admitted, there still exists, in my opinion, sufficient reason for attempting a division of the deposits according to age.1 "Looking over the field, it is undeniable that within many belts of gold-deposits of contemporaneous origin the veins are very similar in mineral composition and metasomatic development. The Appalachian belt of gold-quartz veins contains deposits striking similarity from one end to the other. The Mesozoic gold-quartz veins of the Pacific coast are practically identical in character from Lower California to Alaska, and, moreover, closely related in character to the far older Appalachian belt. . . . On the other hand, scarcely one of the veins, which in so many parts of the Cordilleran region cut volcanic flows of Tertiary age, can be classed as identical with the Pacific coast type of gold-quartz veins. While it is, perhaps, not permissible to say that they represent one type, yet most of them have certain common, peculiar features, constituting a relationship.2 "In conclusion, it may be said that gold-veins of the same age and province usually have the same characteristics. Belts of different age may differ greatly in general features. This is probably due to varying composition of the mineral waters following different periods of eruption." 3 That class of mineral occurrences known as contact metamorphic deposits are found in the United States, British Columbia and Mexico. The principal ore-bearing minerals are copper sulphides and magnetite, which may, however, occasionally carry gold in small quantities. This is especially true of the United States and British Columbia, while in Mexico these deposits are much more common and valuable. In the following pages a brief summary is given of the distribution of gold and silver in rock of the different geological periods. The Older Crystalline Rocks. The crystalline formations of the Appalachians contain gold-bearing veins in an area extending from Maine on the north, to Georgia on the south. Where erosion and glaciation have not removed the decomposed surface rocks, gold is often found in paying quantities either in gossans or placers. The country-rocks are usually schistose or foliated, the gold occurring in lenses of quartz and pyrites. Dikes or igneous rocks seem to have exerted considerable influence in effecting a concentration of the 'T. A. I. M. E., Vol. 33, p. 796, 1903. values. The gold-bearing formations probably belong to some of the Algonkian series. The principal localities in which gold-bearing veins occur in the older rocks are: the Carolinas, Georgia, Maryland, Tennessee and Virginia. Poorer deposits of similar character are found to the northward close to the Canadian line, while still further to the north, in Ontario, Quebec and Nova Scotia quite rich gold-veins occur.1 The Huronian rocks of Lake Superior yield both gold and silver in small amounts, especially when associated with igneous formations. Silver occurs in larger quantities here than in the Appalachians. The crystalline rocks of the Black Hills, Colorado and Wyoming and the granites of Idaho, Montana and Nevada have yielded a large proportion of the precious metals of the older rocks. The Black Hills are Algonkian, while the rocks of Wyoming and Colorado are usually considered as Archæan, although according to Rickard the eruptive. rocks of Colorado are early Tertiary. Here too the deposits are associated with igneous rocks. In the northern Rocky Mountain region the age of the granites is probably largely pre-Cambrian. The granite is eruptive and is traversed by dikes of later igneous material. The unaltered deposits of the districts outlined are largely quartzveins yielding free-gold and auriferous sulphides. According to available evidence the veins are probably pre-Cambrian, and may, therefore, be considered as being the oldest deposits of the North American continent. The following facts are corroborative of such a conclusion: "The Triassic sandstones of the Atlantic coast contain no placer gold; no important gold deposits are found in the Paleozoic rocks of the Appalachian region; Carboniferous conglomerates in Nova Scotia are said to contain water-worn gold of older veins; in the Black Hills the Cambrian conglomerates yield placers of the precious metal." 3 Of all the pre-Cambrian gold-bearing deposits in North America, those of the Black Hills, South Dakota, are undoubtedly of the most economic importance. So extensive has erosion acted upon these veins that it may be said that in most cases, only the lower portions or roots remain. Paleozoic Rocks.- Silver-bearing deposits in sedimentary rocks 'T. A. I. M. E., Vol. 33, p. 800, 1903. 3 T. A. I. M. E., Vol. 33, p. 800, 1903. of the Paleozoic formations are of more common occurrence and of greater value than the gold deposits. In the eastern part of the United States and Canada no valuable deposits of the precious metals have been found in the rocks of this formation, unless some of the gold-bearing schists and the rocks of Nova Scotia prove to be Cambrian. In the Western States the Paleozoic formations are of considerable importance in the production of gold and silver, but usually are most productive when associated with eruptive rocks. Important deposits of silver ores are found in Paleozoic rocks at Eureka, Nevada, where about one-third the value of the ores was gold; at Leadville, Aspen and various districts, where the values are mainly silver in carboniferous limestones; and in several districts in New Mexico, Arizona, Utah, Montana, Idaho and Nevada where silver and lead ores are found in Paleozoic limestones. The Carboniferous quartzites of Ontario; the Wahsatch Mountains, at Bingham; the Oquirrh Mountains of Utah, are all important producers of silver ore, also some lead ores, but little or no values in gold. On the other hand the Cambrian sandstones below the silver-bearing limestones are often gold-bearing. A few gold-bearing deposits occur in siliceous rocks in California, which have been considered as Carboniferous or earlier formations, but are probably Mesozoic. Mesozoic Rocks. The East has no deposits of the precious metals in the Mesozoic sedimentaries. In the West these rocks have yielded the greatest part of the gold output, and in certain localities considerable silver. The western slope of the Sierra Nevada consists of both Paleozoic and Mesozoic rock, but the gold-bearing veins probably predominate in the latter, which are considered as Jurassic and early Cretaceous. Numerous intrusive masses and dikes of diabase and diorite occur in these formations, which have suffered excessive dislocation, being upturned and are covered by uncomformable beds of late Cretaceous. These latter strata are nearly horizontal, and carry some detrital gold, but are free from gold veins. Here, too, so excessive has been the erosive action that only the lower portion of the veins remains. There is no doubt but that thousands of feet of the country-rock have been removed since the veins were formed. Beginning in Lower California, Mexico, a hundred miles or more south of the boundary line, this great belt continues through San Diego, Los Angeles and Kern counties; through the central part of California, where it is developed in great strength; then on to North ern California, southwestern and northwestern Oregon and Idaho. In the latter states it is modified by the appearance of many silvergold deposits, and veins carrying auriferous sulphides without freegold. Covered for a distance by the lava-flows of the Cascades, it again appears in southern British Columbia on Vancouver Island, among other places. Strong development is again attained in the Cariboo district, in Central British Columbia, and it continues through the Omenica, Cassiar and Atlin districts to the Klondike region. Thence, bending westward, it follows the Yukon to the western end of the continent at Nome, on the Seward Peninsula. "The Cretaceous age of this belt is clearly established in California. In Oregon and Idaho a late Mesozoic age is extremely probable. In British Columbia and Alaska the evidence is not positive, and the deposits may possibly, in part, be older." 1 In California, Oregon, and Alaska the veins are often characterized by their smallness and irregularity in occurrence of values, but owing to the excessive erosive action to which they have been subjected, immense and widely distributed auriferous detrital deposits have resulted, and from which a large part of the gold product has been derived. High-grade silver-bearing rocks of Triassic age occur in Western Nevada, while in Utah the silver sandstones are probably of the same age. No intrusives are found in connection with these deposits. Gold and silver ores occur in Cretaceous rock, the veins being closely associated with eruptive rock of various types. Gold-bearing veins are found in igneous rocks at Leadville, in which native gold occasionally occurs with galena ores. In the central and eastern part of the Cordilleran region is an extensive area which contains numerous gold-bearing deposits. It is not definitely known to which geological age these veins belong, but it is probable that many of them were formed subsequent to the California quartz-veins, therefore at the close of the Cretaceous or in the early Tertiary period. This area may be traced from the states of Sonora and Sinaloa, Mexico, in which many gold deposits occur in pre-Cretaceous sedimentary rocks, crystalline schists and granites, through the southwestern part of Arizona, and probably further northward. In Utah the gold-deposits of the Mercur district occur in limestone, in close proximity to intrusions of igneous rock which are probably Cretaceous porphyries. 1 T. A. I. M. E., Vol. 33, pp. 801-2, 1903. At Leadville, Colorado, the gold-bearing formations are sedimentary rocks and porphyry of the Paleozoic age, containing deposits of Cretaceous origin. The same is true of many of the gold-veins of Idaho and Montana, being connected with the intrusion of granites of the Cretaceous period.' Tertiary Rocks. No gold or silver deposits have been found in workable quantities in the sedimentary rocks of the United States. However the igneous rocks of the Tertiary age as they occur in the West, excepting granites, yield large deposits of rich gold and silver ores, and probably those deposits occurring in earlier eruptives did not receive their precious metal content until Tertiary times. The veins of the Tertiary lavas are so common and persistent in certain characteristics that they are usually known as propylitic veins, which refers to the peculiar alteration of the adjacent rock associated with them. These veins carry both gold and silver usually in about equal proportions, although occasionally either metal may occur alone, a more usual occurrence for silver than for gold. The ores are largely siliceous. To these deposits and the occurrence of very rich ore-bodies within them the term "bonanza" has been applied. The bonanzas of the Comstock lode are types. Although not all of these veins are distinctly propylitic, but vary considerably from the characteristic veins of that type, yet they are markedly different from those of the Pacific coast the typical goldquartz veins. The great silver veins, also gold-bearing to a certain extent, of the central plateau belong to the Tertiary period. The gold-silver veins in the andesites of the western slope of the Sierra Madre in Chihuahua, Zacatecas and Sinaloa have proven to be of great extent and value. In Arizona and Nev. Mexico both Tertiary and Cretaceous veins occur, and it is not always easy to distinguish between them. As an illustration of a Tertiary vein the Commonwealth mine of Cochise County, Arizona, may be cited. It occurs in rhyolite and carries about one-third of its value in gold and the remainder in silver. Tertiary veins occur in Oregon, especially in the Bohemia district of the Cascade Mountains, where the gold-bearing veins are found in igneous rocks. The veins of Monte Cristo, Washington, occur in diorite and andesite. 1T. A. I. M. E., Vol. 33, pp. 802 and 803, 1903. |