230 PROCEEDINGS OF THE AMERICAN ACADEMY 



There is a slight Ihic of unconformity between the upper part 

 of the Potsdam or '" Tonto Group " and the Devonian, which rests 

 directly upon the Tacouic, 



The Potsdam rests unconformably on 10,000 feet at least of strata 

 called " Pre-Tonto, or Chuar and Grand Cafion Groups." Tlie first 

 5,000 feet constitute the " Chuar Group," and are formed of sandstone, 

 argillaceous shales, and limestone, resembling in texture and composi- 

 tion the Trenton limestone and Utica slate. 



After very careful researches, prolonged for two months and a half, 

 Mr. Walcott was only able to collect three species of fossils, — a small 

 Discinoid shell, a Pteropod allied to HyoUtkes triangularis^ and an ob- 

 scure Stromatopora. Evidently fossils are very rare, and those found 

 indicate an Infra-Primordial fauna below the zones with trilobites, 

 and recalling the fauna of Conception Bay and Smith's Sound, New- 

 foundland. 



In the 5,000 feet of the "Grand Canon Group" formed of sandstone 

 interposed between "flows of greenstone-lava beds," as Walcott calls 

 them, no fossils have been found ; and one reaches, at last, quartzites 

 crossed bv veins of fjranite. 



To resume. This section of the Grand Canon only shows the base 

 and the summit, that is, the first and last group of the Taconic system. 

 All the middle parts are wanting. As there has been a very marked 

 break, before the deposit of the Potsdam or " Tonto group," we may 

 think that the upper portions have been destroyed by erosions, or are 

 thrown to the right or left of the very narrow region of the Grand 

 Cafion of the Colorado River. 



A little farther north, in the Eureka district, and even already in 

 the ranges of highlands in Eastern Nevada, and of Oquirrh in Utah, 

 the middle portions of the Taconic system have a sujierb development 

 of beds and of fossils, which fully completes the Tacouic in the western 

 regions of North America. 



Eureka. — At Pioche City, the able geologist, Mr. E. E. Howell, 

 has found the zone with Olenelli, or " Georgia Group," well character- 

 ized ; and Mr. S. F. Emmons has recognized this zone with Olenelli a 

 little farther north in the Oquirrh Mountains. Longer and more de- 

 tailed researches have been made in the mining district of Eureka by 

 Messrs. Arnold Hague and C. D. Walcott, and I will give the section 

 of Eureka recently published in the "Abstract of Report on the 

 Geology of the Eureka District, Nevada," by Arnold Hague (Third 

 Annual Report of the United States Geological Survey, 1881-82, 

 ]). 253, Washington, 1884). I give here less than half this section 

 as having relation to the Taconic. 



