222 abstracts: geology 



layers of semifluid clay under the land and the shallow water near the 

 ends of the passes, where this flow is opposed by the comparatively 

 resistant parts of the foreset beds. The tendency to flow is assumed to 

 be due to pressure developed by constant additions of sediment. The 

 mud lumps appear to be the product of flow, because in no other places 

 have such thick bodies of clay been found ; and the facts that they occur 

 almost exclusively near the ends of the passes — most commonly west of 

 them, where probably the principal part of the sediment is being de- 

 posited — and that they are most active during and after tunes of high 

 water seem to be in accord with the hypothesis here presented. 



A. H. Brooks. 



GEOLOGY. — Geology and ore deposits of the San Francisco and adja- 

 cent districts, Utah. B. S. Butler. U. S. Geological Survey Pro- 

 fessional Paper 80. Pp. 212, with map, sections, and illustrations. 

 1913. 



The San Francisco and adjacent districts are located in Beaver 

 County, southwestern Utah, about 180 miles a little west of south of 

 Salt Lake City. The sedimentary rocks of the area range in age from 

 Ordovician to Triassic and in character from quartzites to limestone. 

 A total thickness of about 18,000 feet of sedimentary rocks is present. 



In Tertiary time the area was buried beneath thick flows of lava, and 

 following the extrusion of the lavas large bodies of igneous material 

 were intruded into the sedimentary and extrusive rocks, forming the 

 quartz monzonite stocks of the region. 



Accompanying and following the intrusion of the quartz monzonite 

 there was extensive alteration of the rocks and the deposition of ores. 

 The copper ores occur mainly as replacement deposits in the quartz 

 monzonite and as contact deposits. Lead-silver ores occur as replace- 

 ment veins in the sedimentary and extrusive rocks. These also con- 

 tain some copper and zinc. 



All the ore deposits of the district are believed to have had a common 

 origin, namely, they were the result of the differentiation of the quartz 

 monzonite magma. Such an origin is indicated by the presence of orig- 

 inal sulphides in aplitic rocks and by the presence of coarse pegmatitic 

 quartz associated with the copper ores. The transition from contact 

 deposits to replacement deposits in the sediments is especially well 

 illustrated in the district. 



Since the original ores were formed the deposits have been acted upon 

 by waters leaching downward from the surface, and in many instances 



