1883.] 481 [Crosby. 



within sight of the shore line. Is not this, then, an instance where 

 continental land becomes the bottom of the deep sea ? 



Many writers on this subject seem to consider that the dawn of 

 the Paleozoic era was near the beginning of geologic time, and 

 that, if they can demonstrate that continents and oceans have 

 not changed places since that period, the whole question is set- 

 tled. But this is undoubtedly an erroneous view ; and I think 

 we may safely concede that the continents stand now where they 

 did in Cambrian times without admitting their absolute perma- 

 nence ; and of course the latter is the real point in dispute. If 

 we compare the thickness of the fossiliferous with that of the 

 Eozoic formations, or the degree of metamorphism of the Cam- 

 brian with that of the Laurentian beds, or the amounts of 

 organic evolution before and since the deposition of the earliest 

 Cambrian sediments, it seems difficult to avoid the conclusion that 

 a very large proportion of the time since the appearance of life 

 upon the globe had elapsed before the dawn of the Paleozoic era. 

 The oldest Eozoic rocks have probably never been seen, and below 

 them come the vast thicknesses of Azoic stratified rocks which it 

 is certain were formed after the appearance of the ocean on the 

 globe and before the advent of life. 



Now, if Professor Dana's theory means anything, it means that 

 the continents stood where they now stand during the long 

 Azoic and Eozoic eras as well as during the Paleozoic and later 

 eras. One of the most striking and important facts in historical 

 geology is the profound lithological, stratigraphical, and paleon- 

 tological break observable almost every where between the oldest 

 Paleozoic strata and the underlying Eozoic. What does this 

 signify, if not that there was a general interchange of land and 

 sea at that time ? Before the deposition of the Potsdam sand- 

 stone nearly the entire continent of North America, so far as we 

 know, was dry land, and had been dry land for long ages. How 

 far this land extended beyond the present limits of the continent 

 we have no means of knowing. The Potsdam beds rest every 

 where upon old land surfaces. 



At the beginning of the Cambrian the land subsided and nearly 

 the whole of what is now North America was covered by the 

 Paleozoic sea. How deep this sea became we are unable to de- 

 termine with exactness. Along the eastern shore its bottom 



PROCEEDINGS B. S. N. H. VOL. XXII. 31 DECEMBER, 1SS4. 



