54 GEOLOGY AND MINING INDUSTRY OF LEADViLLE. 



extinction of molluscan life in the former region shall have been thoroughl)' 

 investigated by detailed paleontological determinations, founded upon accu- 

 rate and sj'stematic stratigraphical studies, the assignment of geological hori- 

 zons must be somewhat provisory- and considerable importance must be given 

 to tlie conditions of deposition which prevailed during the Paleozoic era. 



Geologists have observed, both in the East and in the Rocky Mountain 

 region, a certain general sequence in the character of the sediments deposited 

 in the oceans of former geological periods. This sequence has received 

 from Dr. J. S. Newberr}' the name of "circles of deposition," and in a mem- 

 oir* on this subject he has endeavored to prove that in the Appalachian 

 system each great geological period consisted of two extremes, during which 

 the oceanic conditions were such that calcareous .sediments were deposited, 

 separated by an intermediate j)eriod, during which silicious sediment pre- 

 vailed. The former, in a general way, are supposed to have occurred in deep 

 seas and under conditions of comparative quiet, while coarser silicious sed- 

 iments were formed either in shallow waters or during periods when this 

 coarse material would be carried further out towards the middle of the 

 ocean. 



As regards the assumption that limestone may be considered an evidence 

 of deep-sea deposition, it seems that this evidence can be considered only as 

 relative. The limestone depositions in the region under consideration, for 

 instance, were formed in an inclosed arm of the sea, not more than 40 miles 

 in width, and which can therefore have had no very great depth. Mr. John 

 Mui-ray, geologist of the Challenger expedition, informed the writer that 

 the result of their investigations had been to prove that no limestone could 

 be formed in the greatest dejJths of the ocean, and that the area of sedi- 

 mentation is confined to a comparatively shallow and limited belt along the 

 shores of the present continents. While it is probable, therefore, that none 

 of the deposits of the Rocky Mountain region were formed in seas at all 

 comparable in depth to what are classed as deep seas by ocean explorers, 

 the alternations of prevailing silicious and calcareous material in the sedi- 

 ments doubtless represent signiticant changes in the oceanic or climatic con- 

 ditions which prevailed to a greater or less extent over the whole regittn. 

 It is, tlierefore, instructive to observe the parallelism of these conditions in 



