XV.] CAMBRIAN AND SILURIAN IN NORTH AMERICA. 387 



III. CAMBRIAN AND SILURIAN EOCKS IN NORTH AMERICA. 



In accordance with our plan we now proceed to sketch the 

 history of the lower palaeozoic rocks of North America. While 

 European geologists were carrying out the researches which 

 have been described in the first and second parts of this paper, 

 American investigators were not idle. The geological studies 

 of Eaton led the way to a systematic survey of the State of 

 New York, the results of which have been the basis of most 

 of the subsequent geological work in eastern North America, 

 and which was begun by legislative enactment in 1836. The 

 State was divided into four districts, the work of examining 

 and finally reporting upon which was committed to as many 

 geologists. The first or southeastern district was undertaken 

 by Mather, the second or northeastern by Emmons, the third 

 or central by Yanuxem, and the fourth or western by James 

 Hall ; the palaeontology of the whole being left to Conrad, and 

 the mineralogy to Beck. After various annual reports the 

 final results of the survey appeared in 1842. The whole series 

 of fossiliferous rocks known, from the basal or Potsdam sand- 

 stone to the coal-formation, was then described as the New 

 York system. 



At that time the published researches of British geologists 

 furnished the means of comparison between the organic remains 

 found in the rocks of New York, and those then known to 

 exist in the palaeozoic strata of Great Britain. Professor Hall 

 was thus enabled, in his Geology of the Fourth District of New 

 York, to declare, from the study of its fossils, that the New 

 York system included the Devonian of Phillips, the Silurian 

 of Murchison, and the Cambrian of Sedgwick; meaning by 

 the latter the Upper Cambrian, or Bala group, which alone 

 was then known to be fossiliferous. Erom the evidence then 

 before him, he concluded that the Upper Cambrian was repre- 

 sented in the New York system by the whole of the rocks from 

 the base of the Utica slate downward, with the probable ex- 

 ception of the Potsdam sandstone ; while he conceived, partly 



