82 SKETCHES OF CREATION. 



to sponges, with calcareous instead of horny skeletons. In 

 the epoch immediately following this, animal life rose to a 

 slightly higher grade, and unfolded in a great variety of 

 subordinate types. Before the close of the Potsdam pe- 

 riod — before the deposition of the sediments which formed 

 the limestone and marls of Cincinnati, and have given char- 

 acter to the far-famed "blue-grass region" of Kentucky — 

 life had been ushered upon our globe in such richness and 

 variety, that not only had three of the four fundamental 

 plans of animal organization been realized, but all or near- 

 ly all the various classes of the three lower sub-kingdoms 

 had been fairly represented. 



Many extensive regions of the Potsdam sandstone and 

 overlying calciferous sand-rock are, nevertheless, almost, 

 if not quite destitute of the traces of organic existence. 

 Along the south shore of Lake Superior is a sandstone 

 once regarded as belonging to the Potsdam, but probably, 

 in part, of the age of the "Calciferous," in which we search 

 in vain for any of those fossil remains so common in Min- 

 nesota. We find nothing but the imprints of soft sea- 

 weeds (Fig. 29) — things like films of jelly, which have left 

 their imprints upon the coarse rock, and have transmitted 

 to us a knowledge of their existence and nature, while the 

 traces of an army's march are obliterated by the vicissi- 

 tudes of a single season. 



The Lake Superior sandstone, whatever its geological 

 age, is a formation of remarkable interest, both in its re- 

 lations to the basin of the largest lake in the world, in its 

 relations to the world-renowned copper deposits of the re- 

 gion, and, not less, in its relations to some of the finest 

 scenery of the continent. The remarkable interest of this 

 formation was first pointed out by Dr. Douglass Houghton 

 — a name more honored and beloved among the Wolverines 

 than anv other in the lists of science. 



