LE CONTE.l 



C ritu al Periods. 



327 



generalized forms of reptiles, viz., the Theriodonts? It is not prob- 

 able, however, that there was at this time any such general ice- 

 sheeted condition of middle latitude regions as existed in the glacial 

 period. But that there were severe and general oscillations of 

 temperature there is little doubt. 



IV. THE PRE-CAM BRIAN REVOLUTION. 



Last in the order of mention, but first in the order of time, and, 

 so far as physical changes are concerned, by far the greatest of all, 

 is the pre-Cambrian revolution. The Paleozoic era, especially in 

 America, was a time of quiet, of prosperity, and steady evolution. 

 Changes occurred, indeed, during this time, many and great, but 

 not to be compared either in greatness or in wideness to those pre- 

 ceding and succeeding. The revolution at the end, viz., the 

 Appalachian was more conspicuous than that before, only because 

 of the great abundance of life both before and after, and the com- 

 pleteness of the change between. It was therefore equally conspic- 

 uous in the life-system and the rock-system. The pre-Cambrian 

 revolution was even greater in the rock-system (for judging by the 

 universal unconformity it was literally a continental period between 

 two oceanic periods), but there can hardly be said to have been any 

 previous life system with which to contrast that of the Palaeozoic. 

 It is not improbable that farther study will reveal other important 

 divisions, but the meagerness of our knowledge makes it unneces- 

 sary to dwell farther on this. 



Are there any mountain ranges connected with this revolution? 

 — Probably there are. The Laurentide Mountains, of Canada, and 

 the Adirondack's, of New York, are probably examples, but most of 

 such have been swept clean away by erosion and only their fossil 

 bones left in the form of complexly-folded structure of pre-Cambrian 

 rocks. 



These four revolutions, then, I would insist on as marking pri- 

 mary divisions of time. Important movements occur between 

 these, but far less general, and therefore far less effective in chang- 

 ing organic forms, and therefore should form the basis of subordinate 

 divisions only. Three of these great revolutionary periods are gen- 

 erally recognized, but the fourth and last, viz., the glacial, is not 



