278 REVIEWS 



Southward the sediments were broken into faulted monoclines — the 

 typical Great Basin structure. Erosion was active in shaping the moun- 

 tain ranges, especially in the southwest. A second epoch of igneous 

 activity, distinctly separate from the earlier epochs of intrusion, began, 

 probably in Middle Tertiary, as in Nevada, Colorado, Utah, and in 

 general throughout the central West, and andesites and rhyolites, in 

 places 2,000 feet thick, were extra vasated upon beveled sedimentaries. 

 A large Miocene lake covered the upper Rio Grande Valley, in part at 

 least. In this, the Santa Fe marl was deposited. Near the close of 

 the Tertiary, basalts covered this marl, and the eroded older sediments 

 and igneous rocks. 



These eruptions continued during Quaternary times. In early 

 Quaternary, land deposits of coarse gravels filled some of the structural 

 troughs to a depth of 1,000 feet. Basalt was poured over these gravels 

 and smaller flows, perhaps only a few hundred years ago, were extrava- 

 sated at several places. 



The highly acidic potash-rich granites, products of the pre-Cam- 

 brian igneous period, differ greatly from the Tertiary monzonites, quartz 

 monzonites and their lava equivalents, and it is concluded that these 

 two series could not have been derived from a common magma. It is 

 suggested, however, that the Tertiary rocks were derived from the 

 same source and that toward the last a differentiation took place in a 

 magma basin the products of which were basalts and rhyolites. 



The mines of the state, it is estimated, have produced some 35,000,- 

 000 ounces silver, and $30,000,000 gold, besides considerable lead, 

 copper, and zinc. Like the area of maximum of orogenic activity, 

 Assuring and igneous intrusion, the deposits extend soutbwestward, 

 through the state, forming a broad belt about 450 miles long, in which 

 eighty-one mining districts or camps are located. Many types of 

 deposits are represented, among them copper and iron ores in sedimentary 

 beds, fissure veins, mineralized shear zones, lenticular veins in gneiss, 

 replacement veins in limestone, irregular replacement deposits in lime- 

 stone, contact metamorphic deposits and gold placers. At least three 

 epochs of mineralization are represented: (1) Pre-Cambrian, (2) Early 

 Tertiary, (3) Middle and Late Tertiary. There are also, in the "Red 

 Beds" (Carboniferous and later), deposits which are not related to 

 igneous activities and which were formed presumably by cold solutions, 

 in post-Carboniferous times. 



The pre-Cambrian deposits are represented in ten districts. Three 

 types have been recognized, quartz-filled fissures, usually of the lenticu- 



