532 abstracts: geology 



The Cretaceous deposits are divisible into two great series, a lower, 

 the Comanche series, which appears at the surface about the flanks 

 of the Preston anticline in the northwestern part of the area and has 

 an estimated thickness of 800 to 1,000 feet, and an upper, the Gulf 

 series, which has an estimated thickness of at least 3,000 feet and the 

 outcrop of which covers considerably more than half the area. Each 

 of these series is separable into subordinate divisions. The Gulf series 

 is unconformably overlain by strata of Eocene age which appear at 

 the southeast in a relatively small part of the area. In general the 

 strike of the strata is parallel to the inner margin of the Coastal Plain, 

 and the dip is coastward from this margin at rates ranging from 30 

 feet or less to 80 feet or more to the mile. A considerable departure 

 from the prevailing regularity in strike and dip occurs in the north- 

 western and central parts of the area, in connection with the Preston 

 anticline. 



The Cretaceous deposits consist of sand, shaly clay, calcareous shaly 

 clay, limestone, and chalk. Pleistocene alluvial terrace deposits largely 

 conceal the Cretaceous formations in a broad area. 



R. W. Stone. 



GEOLOGY. — Geology and ore deposits of the Yerington district, Nevada. 

 Adolph Knopf. U. S. Geol. Survey Prof. Paper 114. Pp. 68, 

 pis. 5, figs. 12. 1918. 



The Yerington district in western Nevada is, next to Ely, the most 

 productive copper district in the State. The oldest rocks of the dis- 

 trict consist of andesites, keratophyres, and limestone, with subor- 

 dinate shale, quartzite, and gypsum, all of Triassic age. They were 

 intruded in post-Triassic time, probably early in the Cretaceous, by 

 granodiorite, which was followed by quartz monzonite. These intru- 

 sions intensely metamorphosed the rocks they invaded and converted 

 large areas of them into lime-silicate rocks. After this metamorphism 

 the region was cut by numerous dikes of quartz monzonite porphyry. 

 Faulting then ensued, and along the faults ore-forming solutions 

 rose and produced the copper deposits to which the district owes its 

 economic importance. 



The Tertiary rocks, resting with marked unconformity on the Meso- 

 zoic group, are chiefly volcanic and are at least 7,000 feet thick. They 

 fall into three major groups which are separated by two well-marked 

 unconformities. The lowest subdivision consists of quartz latite, 

 rhyolite, and andesite breccia; and it is probably the correlative of the 

 Esmeralda formation of Upper Miocene age. The middle subdi- 



