252 proceedings: geological society 



THE GEOLOGICAL SOCIETY OF WASHINGTON 



The 291st meeting was held in the lecture room of the Cosmos Club 

 on February 10, 1915. 



INFORMAL COMMIJNICATIONS 



■ S. R. Capps: A71 unusual exposure of a great thrust-fault. A remark- 

 able exposure of a great thrust-fault in the canyon of Nizina River, 

 Alaska, near the mouth of West Fork, was described. A photograph 

 was exhibited, showing a great cliff, about 5,000 feet high, in which 

 the fault is developed in almost diagrammatic perfection. The rocks 

 involved are the Nicolai greenstone (probably Carboniferous), and the 

 Triassic Chitistone limestone and McCarthy shales. A displacement 

 of about one-half mile has taken place; the Chitistone limestone has 

 been reduplicated to almost twice its normal thickness. The crumpling 

 of the overridden beds and the drag of the overriding beds were well 

 brought out. 



D. F. Hewett: Calculation of the thickness of strata represented in 

 a series of outcrops of varying dip. A formula was presented for cal- 

 culating the thickness of folded beds between two horizons of widely 

 diverse inclination. The formula was based upon the assumption of 

 parallel folding, and the degree of accuracy of the various factors in- 

 volved in the calculation was stated. 



Discussion: M. I. Goldmann inquired more particularly as to the 

 basis on which the calculations had been made, and this was explained 

 by Hewett. C. E. Lesher mentioned a graphical method which had 

 been proposed for the solution of the problem. 



REGULAR PROGRAM 



F. C. Schrader: Some features of the ore deposits in the Santa Hita 

 and Patagonia Mountains, Arizona. The deposits are exposed in about 

 a thousand mines and prospects scattered throughout the mountains. 

 They occur in two sharply contrasted groups that differ considerably 

 in age and represent two distinct periods of mineralization. The earlier 

 and more valuable group occurs in association with the Paleozoic sedi- 

 mentary rocks and the Mesozoic granular intrusives, and is referred 

 to the late Mesozoic epoch of metallization. 



The later group belongs to the great group of metalliferous deposits 

 formed near the surface in the Tertiary volcanic rocks throughout the 

 West by ascending thermal solutions, and in genetic connection with the 

 associated rocks. It is referred to the Miocene and to the late Tertiary 

 epoch of metallization. Its deposits occur chiefly as copper-, silver-, 

 lead-, and gold-bearing fissure veins, in which the filling is mainly 

 quartz, fluorite, and calcite. 



The older deposits contain the metals or minerals of gold, silver, 

 copper, lead, zinc, tungsten, and molybdenum. They were apparently 

 deposited at considerable depths, chiefly by ascending thermal solu- 

 tions that circulated as a close after-effect of the intrusion of Mesozoic 



