USGS Professional Paper 144 pp 141-146



     To recapitulate, five agencies that might effect the deposition of copper from ascending solutions have been considered - (1) cooling, (2) relief of pressure, (3) oxidation, (4) dilution, (5) reduction of acidity. Three of these agencies require specific constituents in the solution - the first, cuprous sulphate; the third, potential cuprous sulphide; the fourth, cuprous chloride. The third also demands an oxidizing environment for the incoming solutions, and the fifth is based on an environment that will neutralize acidity. Two of the agencies, however, may probably be excluded from consideration at once on account of the difficulty of defining the conditions with the necessary degree of certainty. These are the second and fifth.

     The only application of the second agency that appears of importance as a possible cause of the deposition of native copper is the escape of certain acid gases, particularly carbon dioxide, which would leave the solutions more alkaline. This would tend to assist the deposition of copper by ferrous compounds, but other factors, such as oxidation, might tend to make the solutions more acid. It is extremely difficult to feel sure of the direction of the variation of acidity, so that it seems best to dismiss the fifth agency and with it the second from consideration. Another reason for doing so is the field evidence that the precipitating solutions seem to have dissolved ferric oxide rather than to have deposited it as required by the fifth agency for the deposition of native copper, and still a third reason is that sulphur compounds are not considered or their general absence explained by either the second or the fifth agencies.

     The first and fourth agencies are so general that they might apply almost anywhere. They also require the presence of specific compounds, cuprous sulphate and cuprous chloride, respectively, and to call on them as explanations of a rather unusual type of ore, that of native copper, obviously requires some additional feature, such as, possibly, the large scale of the field. But this is as unwarranted as it is unnecessary. Thus the third agency is left as the only alternative explanation.

     The third agency of deposition seems to fit the Lake Superior district best because the specific influence of ferric oxide in the gangue rocks is taken into account in determining the character of the copper mineral deposited, and it is probably unnecessary to delimit the character of the solutions further than to say that they were a potential source of cuprous sulphide and, so far as the necessities of the chemical experiments suggest, somewhat acid. The last statement might possibly imply nothing further than solutions containing large quantities of carbon dioxide. The general absence of sulphides is explained by the oxidizing action of ferric oxide, and the deposition of copper is explained by cooling of the resulting solutions at least, if not by a direct chemical reaction, which appears probable at moderately high temperatures.,


     In the preceding pages the several views regarding the origin of the native copper deposits of Michigan that have been advanced by previous investigators have been outlined, and the views held by the present writers have been elaborated in detail. A brief general summary of the argument may help the reader to judge the relative merits of all these proposed explanations. It will be emphasized in this review that the theory arrived at by the writers differs outstandingly from those of some others in postulating that the copper was deposited from ascending rather than descending solutions and was precipitated by oxidation of the ore solutions rather than by reduction. As in the fuller statement, origin, transportation, and deposition will be considered in turn.


     If the deposits were formed by descending solutions the copper must have been derived from the general mass of the Keweenawan lavas, which are known to contain small amounts of copper - amounts comparable to those contained in similar rocks at many places. So far as known, this copper is pretty uniformly distributed. It is regarded by those who favor the view of descending waters as an ample source for the copper deposits. Those who favor the view of ascending solutions must also recognize this minutely disseminated copper as quantitatively sufficient, but they see no evidence that it has been concentrated. It is still contained in the traps to the extent of a few hundredths of 1 per cent. Is it reasonable to suppose that solutions would dissolve out a constituent present in minute quantity and concentrate it to the amount of 50 to 100 times without having a notable effect in dissolving the minerals that are present in much larger quantity?

     Those who favor the view of an ascending origin consider the Duluth gabbro, which is believed to underlie the whole region, as the source of the copper. Copper in small amount is associated with the small offshoots of this igneous body, and to those whose experience has led them to look upon intrusive bodies as a source of ore deposits this source seems adequate, favorably located, and probable.


     Gravity circulation of solution is regarded as the transporting agent in the theory of descending origin. Doubt is thrown on the sufficiency of that method by the very slow rate of gravity circulation as indicated by the dryness of the deep levels of the mines and by the difficulty of conceiving a gravity circulation as operative far below sea level, as it must have been under conditions at all like the present. Moreover, the position of many of the ore shoots beneath relatively impermeable rocks seems inconsistent with deposition by descending solutions.

     The view of concentration by diffusion avoids some of the difficulties of gravity circulation but meets others in explaining why copper was not concentrated equally in all lodes of similar physical character and chemical composition and why the ore occurs so definitely in shoots.

     In the theory of ascending origin the medium of transportation is regarded as the solutions given off by the crystallizing magma of the Duluth gabbro. These were heated liquids or gases and therefore very mobile and were under high pressure, which could force them in quantity through rocks where gravity circulation would be practically nil. The solutions either entered the lodes by the direct connection of the downward extension of the lodes with the igneous mass or were led into the lodes through faults or fissures that extended to the igneous mass. This explanation seems to furnish an entirely adequate means of transportation and one entirely similar to that believed to have been active in the formation of most primary copper deposits.


     The theory of descending origin assumes that the copper was carried as an oxidized compound and was deposited through reduction by ferrous iron.

     The conglomerates and amygdaloids were very rich in ferric iron and poor in ferrous iron long before the copper was introduced. Moreover, the alteration of the rock associated with the copper indicates that when the copper was deposited the ferric oxide was actually reduced and ferrous oxide added - a fact which shows pretty clearly that reduction by ferrous oxide was not the process that formed the copper.

     The theory of ascending origin assumes that the solutions were such as ordinarily deposit sulphides and were essentially reducing and that if they had encountered rocks of the ordinary composition they would have deposited sulphides. But because these solutions encountered rock with a high content of ferric iron there were chemical changes before precipitation. Ferric iron was reduced, sulphur was oxidized, and native copper was formed. The lodes rich in ferric iron would be the places most favorable for the precipitation of copper.


     There are throughout the world other deposits of native copper in which the copper appears to have been deposited as native metal and not to have been formed by surface oxidation of some other mineral, such as chalcocite. The chief examples of such deposits occur in Coro Coro, Bolivia; the Triassic areas of New Jersey and Connecticut; Cape d'Or, Nova Scotia; Oberstein, Germany; Commander Island, Russia; the Faroe Islands; Sao Paulo, Brazil; Upper Serbia; the Copper and White River districts, Alaska; the Comobabi Mountains, Arizona; Nova Zembla, Russia; and Coppermine River, Canada. The conditions of occurrence at these localities are set forth below:

     Coro Coro, Bolivia.41 - At Coro Coro occur the largest of a series of similar copper deposits that lie in a zone that extends across the Bolivian high plateau. The copper is found in the Puca red sandstone, of Cretaceous age, in both vein and bedded deposits. Mineralization has occurred at a number of closely spaced horizons. The dominant copper mineral is the native metal, although the sulphide chalcocite and the arsenide domeykite are sparingly developed. Native silver is also found associated with the copper. Oxides of copper occur at the surface but give way to the native metal, which persists at least to 380 meters, the greatest depth attained by the present workings. The sandstone, which away from the ore has a distinctly red color tone due to iron oxide or hydroxide, is bleached in the vicinity of the ore. The copper is thus usually surrounded by a halo of whitish or greenish rock that grades into the red which is the prevalent color of the formation away from ore.

     At Coro Coro we evidently have the essential conditions to produce native copper from ascending sulphide solutions. None of the complications of lava flows as possible sources of copper, of ferrous minerals as possible reducers of oxidized copper solutions, nor of zeolites, prehnite, datolite, feldspar, or other associated minerals, that have served to cloud the situation in Michigan, are present. The evidence indicates that the solutions were ascending and that they were of the type that ordinarily would deposit sulphides. They encountered a highly oxidizing environment (red sandstone) and deposited native copper and native silver with considerable copper sulphide (chalcocite) and some arsenides. The rock surrounding the metals was bleached, and sulphates were deposited. The conditions are essentially those that are found in the conglomerate and sandstone lodes of Michigan.

     Triassic of New Jersey.42 - The Triassic of New Jersey consists mainly of characteristic red shale and sandstone but comprises also coarse conglomerate, black argillite, and gray or green flagstone. Both intrusive and extrusive rocks resembling the Lake Superior basalts in composition are also present. The main intrusive mass is that forming the Palisade diabase. The ore occurs in veins, which cut both igneous and sedimentary rocks, and disseminated in the sediments either just below the base of the effusive rocks or close to the dikes.

     The dominant copper mineral of the veins in the Palisade diabase is chalcopyrite. In the sediments close to intrusive rocks the copper occurs chiefly in chalcopyrite, bornite, and chalcocite, the native metal being absent. Away from the intrusive rocks native copper is the dominant ore mineral, although a little chalcocite is usually present.

     Most of the native copper occurs in the red shale and sandstone just beneath the effusive rocks that constitute First Mountain. The deposit in the American mine, near Somerville, N. J., is typical of these occurrences. The ore bed is a purple rock with a texture between that of a fine-grained sandstone and a shale. This bed, which has been explored for a depth of 1,300 feet down the dip, is sparingly mineralized over a maximum thickness of 2 1/2 feet. Wherever the copper occurs in this bed the rock has lost its purple color and is blanched to a pale gray or greenish white. Chalcocite is invariably associated with the native copper, which it apparently follows in age.

     Triassic of Connecticut .43 - In the central part of Connecticut basaltic flows, some of which show red amygdaloidal tops, are interbedded with the red Triassic sandstone and shale. At the Newgate prison, Simsbury, disseminated bornite occurs in the sandstone, whereas the near-by trap carries native copper. At Meriden a core of native copper is inclosed by a shell of chalcocite, thus indicating the chalcocite to be the younger mineral.

     Cap d'Or, Nova Scotia .44 - The native copper at Cap d'Or, Nova Scotia, according to Sir William Dawson, forms masses ranging from some several pounds in weight down to the most minute grains in the veins and fissures that traverse the trap, interbedded with the red Triassic sediments. The trap is amygdaloidal and carries various zeolites, such as analcite, natrolite, and chabazite. The deposits were examined by A. C. Lane, who found them strikingly like those at Lake Superior. Lane notes that the copper is found in veins that cut the lavas.

     Oberstein, Germany. - At Oberstein, Germany, amygdaloidal basalts are interbedded with red sandstones and shales of Permian and Triassic age. In the amygdaloidal cavities of the basalts and in the red sediments a little disseminated native copper occurs. Fissures cutting the traps also contain narrow veins of chalcopyrite, with which are associated pyrite, calcite, prehnite, a boron mineral (datolite?), and analcite.

     Commander Island, Russia .45 - The deposits of Commander Island, Russia, are described at length by Morozewicz. The island consists of Tertiary effusive rocks belonging to the soda rhyolite family, overlain by andesitic and basaltic tuffs and breccias. The effusive and elastic rocks are cut by basaltic and andesitic dikes. The basaltic tuffs are described as being of a gray-green color and are cut by basaltic dikes. The copper occurs in both the tuffs and the dikes and is associated with zeolites. The order of mineral formation is given as iron oxide, calcite, analcite, and wire copper.

     Faroe Islands .46 - The Faroe Islands consist of late Tertiary basalts with which are associated red tuffs and volcanic breccias. Between some of the flows are red shaly layers carrying plant remains. Dikes and sills are rare, and marked faults are missing. The effusive rocks are typical amygdaloidal basalts, in which the amygdules are rich in zeolites. Native copper appears sparsely disseminated in the tops of the youngest as well as in the oldest of these flows. The copper-bearing amygdaloidal portion of the flows is of a violet-gray color. Associated with and apparently later than the copper are the zeolites, stilbite, and heulandite. The copper also occurs in the interstices of the breccias intimately associated with the cementing zeolites. At Suderoe the copper occurs in the amygdaloidal portion of a dense black trap. The amygdaloidal cavities carry stilbite, mesolite, heulandite, and a fluorine-bearing apophyllite, as well as copper; the copper seems to be the oldest of these minerals and is found upon the walls of the cavities. At Vaag the copper occurs as thin plates in a dark-brown tuff.

     Sao Paulo, Brazil .47 - In the State of Sao Paulo, Brazil, diabasic rocks occur as dikes and stocks and as flows interbedded with Permian sandstone and shale. The flows are amygdaloidal, their vesicles having been filled with zeolites, chalcedony, and calcite. At Sorocabana the brownish-black diabase shows flattened open spaces that are now lined with chalcedony and filled with a hydrous iron silicate. Native copper occurs at the boundary between these two minerals.

     Upper Serbia.48 - The native copper of Upper Serbia occurs in the vuggy openings of a hornblende andesite, which shows dacitic phases, together with chabazite, heulandite, stilbite, apophyllite, and calcite. The copper is incrusted with the zeolites. It is of the leaf variety, although crystals are also found. The copper occurs also in certain highly propylitized portions of the andesite; it is here associated with chabazite, heulandite, stilbite, opal, chalcedony, and calcite, which are younger than the copper.

     White River, Alaska.49 - The copper minerals on White River, Alaska, occur in interbedded effusive and pyroclastic rocks of Carboniferous age. Both sulphides and native copper are present. The lavas are slightly altered basalts of dark-brown, reddish, and green colors. The contact between two flows is as a rule easily determined because of a marked color difference. The copper minerals include both sulphide (usually chalcocite) and native copper; they are generally confined to the upper or amygdaloidal portions of the flows, although they also occur in veins and stringers. Associated with the copper minerals are zeolites, prehnite, quartz, and calcite. At the head of the Middle Fork of White River the country rock consists of stratiform basalts intercalated with beds of breccia and brick-red tuff. The native copper which occurs here is apparently limited to a certain definite volcanic sheet-a reddish lava, which is in places highly amygdaloidal. For 200 feet along the outcrop of the amygdaloidal rock metallic copper intergrown with prehnite, calcite, and zeolites can be found here and there in encouraging amounts. The copper occurs as irregular reticulating masses of metal several inches long and as small lumps and minute particles embedded in the minerals that line or fill vesicles in the lava flow.

     Copper River district, Alaska.50 - In the Copper River district of Alaska the copper occurs in the preTriassic basaltic and andesitic rocks, which attain a thickness of more than 3,000 feet. The native metal occurs at different horizons in different parts of the district, but nowhere in encouraging amounts.

     On Glacier Creek, a tributary to Chitistone River, the native metal, associated with chalcocite, occurs in a greenstone filled with black amygdules. Masses of native copper weighing several pounds are found, but the metal is present chiefly as small specks in the greenstone and the black amygdules and as paper-thin sheets or leaves.

     On Fall Creek native copper occurs in a shattered grayish amygdaloidal greenstone. It forms small particles in both the altered and the seemingly unaltered greenstone and also in small veinlets of calcite and quartz.

     On Nugget Creek a very little native copper occurs in the reddish, highly epidotized amygdaloidal portion of a basaltic flow. The copper is intimately associated with calcite and appears to be later than the chlorite, epidote, quartz, and prehnite.

     Nova Zembla, Russia.51 - The rocks of the Nova Zembla islands are interbedded limestones, conglomerates, and basaltic lavas. The basalt is in part intensely epidotized and is also cut by calcite and epidote veins. Both sulphides and native copper are disseminated in the amygdaloidal portions of the basalt and in veins. The native copper occurs in a red-brown brecciated top of gray-green or black basalt, which may locally be bright red owing to the presence of iron hydrate. The copper is deposited in the epidotized rock. Copper also occurs in a green augite porphyrite. The rock is strongly epidotized, and usually along with the copper this epidotization is marked, although abundant epidote does not always signify copper.

     Zwickau, Saxony.52 - Native copper occurs in the red beds at Zwickau, Saxony. On both sides of minute copper veinlets the red rock is bleached yellowish owing to the reduction of the ferric hydroxide that colors the rocks.

     Algodones, Chile.53 - In the Mercedes mine, Algodones, Chile, a Mesozoic gray sandstone is cut by a diabase porphyry dike. In fractures in the dike there is a little native copper, with which a little native silver is associated. Native copper and cuprite in association with calcite and quartz replace propylitized portions of the rock, which is strongly impregnated with native copper. No ore appears to make in the sandstone. The amygdules of this dike rock contain calcite and delessite (?), and the mineral algodonite also occurs at this locality.

     Comobabi Mountains, Arizona.54 - In the eastern portion of the Comobabi Mountains, Arizona, there are lenses of altered greenish lava as much as 100 feet long, lying irregularly on lava. In this greenish lava quartz occurs as an alteration product of a red amygdaloidal basalt, and with the quartz in places there is native copper and cuprite. The altered rock that carries the copper is thoroughly epidotized and silicified. The amygdules usually show chlorite, epidote, quartz, and copper. The epidotized rock is cut by minute veinlets of quartz, epidote, and albite.

     Permian "Red Beds" of the Southwest.55 - In the "Red Beds" of the Southwest copper ore occurs in bituminous clay slate and marl in nuggets, nodules, or groups of irregular pockets, as carbonates, silicates, and siliceous carbonates. At Judge Kerr's farm, near Archer City, Tex., the green copper ore occurs in whitish-blue to dark-gray clays. At the Ball mine, about 7 miles northwest of Archer City, the ore consists of nodules and nuggets in a stiff white to gray bituminous clay slate or marl. This clay slate is interbedded with iron-rich clay and conglomerate. At the Isbell lead, half to three-quarters of a mile southeast of the Ball mine, the ore occurs as pseudomorphs after wood or as irregular lumps of black and green silicates in a slightly bituminous clay slate and marl.

     In Oklahoma56 the ore occurs as sulphide and as the native metal in the "Red Beds," consisting of sandstone and shale of a prevailingly red color. The sandstone is fine grained and ranges in color through white and yellow to red. The sulphide has locally replaced the woody material at a definite horizon. Among the sulphides observed were chalcocite and chalcopyrite. Azurite, malachite, and chalcanthite are also present. At Coldwater, Okla.,57 the copper occurs as very thin sheets in a bed of red shale.

     In New Mexico58 the ores occur not necessarily in the red beds that give rise to the name, but in the light-colored sandstones which are interbedded with them. The most conspicuous ores are malachite and azurite, but these are merely oxidation products of chalcocite. With the chalcocite are small quantities of bornite, pyrite, and chalcopyrite. Near Estey the chalcocite ore replaces the calcite cement of a 500-foot bed of red sandstone. The Copper Glance mine has sulphides, silicates, and carbonates of copper in a whitish, yellowish, or reddish sandstone. With the chalcocite a little hematite is associated. Of the copper minerals in the ore worked, about 60 per cent was chalcocite and about 40 per cent carbonate. About 5 per cent of the total copper content in most of the ore is present as native copper.

     Montana. - Billingsley and Grimes59 describe an occurrence of native copper at Copper Hill on Baggs Creek, east of Deer Lodge, Mont. Copper Hill consists of a series of lava flows, basaltic at the base but andesitic toward the top. "Native copper in appreciable amounts is restricted to limited lenses within these flows. The metal occurs in the groundmass, in the augite phenocrysts, and in the amygdaloidal cavities, in the latter case with quartz, calcite, and zeolites (rare)." The authors consider the deposits due to a concentration by comparatively cool waters of copper originally widespread as a constituent (0.02 per cent or less) of the original rock.

     Arctic Canada. - It has long been known that native copper occurs on the mainland and islands of northern Canada over a wide area. These deposits have been visited by a number of men, but the best accounts of them are one by Dr. James Douglas,60 based on an examination and reports by George M. Douglas, Lionel Douglas, and August Sandberg, who examined the Coppermine River region in 1911, and one by J. J. O'Neill,61 on the Arctic coast west of Kent Peninsula. The report by O'Neill reviews previous literature and contains maps showing what is known of the geology and geography of the region. The native copper occurs in a series of basaltic lavas interbedded with basic or amygdaloidal conglomerate.

     The regions about which most is known are those near Coppermine River and Bathurst Inlet. In the Coppermine River region, according to Sandberg, red  rock occurs, and the rock associated with the copper is much altered. These conditions resemble those found in Michigan.

     In bed No. 2 the rock, where exposed, has been very much altered in some places to epidote and a crumbling mass of light-colored rock, in which nearly all the amygdules contain copper carbonates. Native copper in the form of chips and flakes is fairly abundant in this altered rock.

     In the Bathurst Inlet region the copper occurs disseminated in the traps, in the amygdaloids, and in fissures. Copper sulphide, principally chalcocite, has replaced dolomite underlying basalt, and chalcopyrite and chalcocite are disseminated in some of the sills or dikes of the region. Chalcocite also occurs in fissures in both regions.

     O'Neill sums up the evidence on the origin of the deposits as follows:


     1. The apparently uniform distribution of native copper in individual flows of lava of large extent, and the occurrence of such flows throughout so extensive a district.

     2. The copper is abundant in some flows and apparently absent from others.

     3. The copper occurs minutely disseminated throughout the dense, massive part of the flows, as well as in the upper amygdaloidal parts.

     4. In many places the rocks containing the copper are apparently fresh and unfissured.

     5. In many instances copper occurs in the dense groundmass of a flow, while apparently none occurs in the amygdaloidal portion of the same flow.

     6. Copper sulphides occur disseminated through massive sills of diabase, which probably came from the same magma as did the surface flows.

     7. No enrichment of native copper has occurred in flows cut by sills of diabase, although the flows contain native copper and the sills sulphides of copper.

     8.62 On the Coppermine River conglomerates interbedded with copper-bearing lava flows carry native copper in the contained pebbles, but copper was not observed in the matrix by the Douglas party. The copper therefore must have been in the amygdaloid before the immediately overlying conglomerates were deposited.


     1. Specimens of the copper-bearing flows examined under the microscope show that minute grains of native copper replace the matrix or some of the minerals of the rock.

     2. Native copper forms the outer edge and in some cases the center of amygdules, and in some instances replaces other minerals of the amygdaloid filling.

     3. In places native copper occurs in thin fissures and in veins in the flows, and at some places the copper was found to be more abundant nearer minute fissures than through the rest of the rock.

     4. Chalcocite occurs in some of the veins in the flows.

     5. Dolomites immediately underlying the copper-bearing basalts in many places have been partly replaced by chalcocite. The chalcocite is intimately mixed with covellite, so that it is probable that secondary enrichment has taken place to some extent.

     6. At one place, on Iglor-u-allik Island, copper occurs about the contact of two of the flows of basalt. The lower foot of the upper flow contains considerable native copper, but no copper was seen throughout the rest of it.

     7. A specimen of native copper in conglomerate was brought from the Coppermine River district to Dr. J. A. Allen, of the University of Alberta. The writer was shown this specimen and was immediately struck with the fact that the native copper in this case had replaced most of the matrix around the pebbles of amygdaloid.


     41 Singewald, J. T., Johns Hopkins Univ. Studies in Geology No. 1, 1922.

     42 Lewis, J. V., State Geologist New Jersey Ann. Rept. for 1908, pp. 131-184,1907.

     43 Foye, W. G., personal communication, 1922.

     44 Dawson, William, Arcadian geology, 1878.

     45 Morozewicz, J., Com. géol. Mém., new ser., livr. 72, p. 44, 1912.

     46 Cornu, F., Zeitschr, prakt. Geologie, 1907, p. 321.

     47 Hussak, E., Ueber das Vorkommen van gediegen Kupfer in den Diabasen von São Paulo: Centralbl. Mineralogie, 1905, pp. 333-335.

     48 Lazarevic, M., Zeitschr. prakt. Geologie, vol. 18, pp. 81-82, 1910.

     49 Knopf, Adolph, Econ. Geology, vol. 5, p. 247, 1910.

     50 Moffit, F. 11., U. S. Geol. Survey Bull. 345, pp. 143, 188, 1908.

     51 Voit, F. W., Zeitschr. prakt. Geologie, vol. 21, p. 42, 1913.

     52 Neues Jahrb., 1873, p. 84.

     53 Möricke, W., Die Gold- Silber- and Kupfer-Erzlagerstätten in Chile and ihre Abhängigkeit von Eruptivgesteinen: Naturf. Gesell. Freiburg im Breisgau Ber., Band 10, p. 180, 1897.

     54 Joralemon, I. B., report to Calumet & Arizona Copper Co.

     55 Schmitz, E. J., Am. Inst. Min. Eng. Trans., vol. 28, pp. 97-108,1896. 

     56 Tall, W. A ., Econ. Geology, vol. 5, pp. 221-228, 1910.

     57 Haworth, Erasmus, and Bennett, John, Geol. Soc. America Bull., vol. 12, pp. 2-4,1900.

     58 Lindgren, Waldemar, and others, U. S. Geol. Survey Prof. Paper 68, 1910.

     59 Billingsley, Paul, and Grimes, J. A., Ore deposits of the Boulder batholith, Mont.: Am. Inst. Min. Eng. Trans., vol. 58, p. 293, 1918.

     60 Canadian Min. Inst. Trans., vol. 16, pp. 83-144, 1913.

     61 Canadian Arctic expedition, 1913-1918, Rept., vol. 11, Geology and geography, pp. 1A-107A, 1924.

     62 See also No. 7 under "Facts favoring an epigenetic origin."

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