421 



stratigrapliical break between the two sets of strata must furnish 

 the strongest presumption that they appertain to distinct geological 

 systems. In maintaining this as the natural inlerence from the 

 facts, it is by no means intended to imply that the subjacent so- 

 called azoic rocks have from the beginning contained no traces 

 of organic life, or even that in the progress of research distin- 

 guishable fossils may not be discovered in some of their divisions. 

 But, judging by the usual law which associates great changes of 

 fauna with great and wide-spread discordance of stratification, it 

 would seem most probable that the organic forms, should such be 

 found in these underlying rocks, instead of belonging to the Pri- 

 mordial fauna as heretofore defined, would form a separate group 

 equally marked and peculiar, and claiming for itself a distinctive 

 name. 



In view of these considerations, and of the facts thus far known. 

 Prof. Rogers could not perceive any valid reason for maintaining 

 that the primordial series of deposits extends in this country below 

 the base of the Primal or Potsdam group. He knew of no satis- 

 factory evidence of the occurrence of fossils of any kind in the 

 rocks found unconformably below this group, notwithstanding the 

 vast extent through which its outcrop had been traced, and he 

 could, therefore, see no ground for the opinion expressed by Mr. 

 Marcou that the Potsdam sandstone is the cover rock " capping a 

 system of 30,000 feet of fossiliferous strata." 



Referring to the fossils of primordial character discovered at 

 Georgia and Point Levi, Prof. Rogers alluded to observations 

 which he had made some years ago in Western Vermont, in 

 which he had traced the group of reddish arenaceous and cal- 

 careous rocks overlying the characteristic Hudson River strata, 

 northward from the flank of the Snake Mountain to near the 

 Canada line. Although he had not visited the spot in Georgia 

 where the trilobites are found, the uniform and moderate east- 

 ern dip in the belt bordering the lake and embracing this lo- 

 cality impressed him with the belief that these trilobite layers 

 are included in regular sequence in this upper group. The 

 observations of Prof Hall, and the recent sections of Prof. Hitch- 

 cock in the Vermont survey, had conducted to the same result. 

 As to the age of the Point Levi rocks and the true place of the 

 fossils contained in the conglomerates, he must rely upon the high 

 authority of Sir Wm. Logan's observations. 



