SCIEXTTFIC LeCTVBES. 81 



this map, in its clifFerent colors as indicating geological formations of 

 certain ages, was one vast expanse of ocean. 



At a later period this ancient ocean bed was raised above the sea 

 level, constituting the earliest known form and condition of the North 

 American Continent. The limits of this Lanrentian area marked 

 the earliest coast line with which we are acqnainted. Along this 

 shore line, and throughout a considerable portion of its extent, were 

 accumulated large areas of sand and pebbles forming conglomerates, 

 and fine sand giving rocks of quartzite. Still the outline of the early 

 continent was essentially maintained by the rocks of the Laurentian 

 system. 



Above all these, and constituting the beginning of the next great 

 geological period, there came, along the entire coast line, an extensive 

 deposit of coarse sand and pebbles ending in fine sand. This formation 

 occurs on the northeast in Newfoundland, and extends southward 

 throughout the central portion of the country, and westward even to the- 

 Rocky mountains. This was the earliest sand beach known, skirting 

 the ancient American continent, which then had no further extension 

 southward than you see upon this map ; limited on the south by the 

 latitude of the great lakes, except small detached portions which may 

 have been islands in the ancient sea. This entire belt, skirting the- 

 old continent as a sand beach, was converted at last into sandstone,, 

 known in our modern geological nomenclature as the Potsdam 

 sandstone. Let me say here that this northern portion of the conti- 

 nent has never, so far as we have evidence, been entirely beneath 

 the water since this sand began to be deposited. Some portions- 

 have been eroded, because this area is marked by hills and 

 valleys the same as other parts of the continent, and some of the- 

 lower valleys have been invaded by the ocean, to a small extent, but 

 never the continent as a whole. Above this sandstone we have a 

 series of limestones, and other rocks ; but in order that you may better 

 understand the details, I will refer to this other map which shows 

 more distinctly the order of succession, as it has been developed in 

 the course of the last twenty-five years in the geological surveys which 

 have been made ; first in the State of New York, and subsequently 

 in other parts of the United States, and in Canada, where the 

 researches have developed the same system of rocks we have found 

 here. [Reference to geological map and sections.] 



To illustrate the fact that these formations liave all been deposited 

 beneath water, I have liere a specimen, covered with the shells of 



[Inst.] 6 



