THE FRONT OF THE PROCESSION OF LIFE. 



bottom of the group, was long regarded as the oldest fos- 

 siliferous rock in America. It is certainly not far from the 

 lowest horizon of the primeval cemeteries which hold the 

 dust of the first denizens of our planet. This sandstone is 

 sometimes whitish or grayish, but often of a dull red color, 

 and sometimes slaty ; and except within the area covered 

 by the St. John's group, it is found resting upon the up- 

 turned edges of the Eozoic strata. Observe that it is a 

 sandstone. Now we know that in the waters of the pres- 

 ent day, sands are accumulated only in comparatively tur- 

 bulent and shallow regions. In calmer and deeper waters, 

 the sediments are necessarily finer, as only the finest par- 

 ticles can be transported by the slowest moving currents 

 (compare Fig. 15). Moreover, many a layer of this ancient 



sandstone, when uncov- 

 ered to the light, pre- 

 sents us with veritable 

 ripple marks — such as 

 the waves are making 

 to-day in the fine sand 

 of the shallow water 

 near the beach — sand- 

 ripples which have been 

 preserved unmarred for 

 millions of years, and 

 unite with other proofs 

 that the bottom of the 

 Protozoic sea was not 

 beneath the reach of the 

 agitations of its surface. 

 This interesting sand- 

 stone Was first atten- 

 tivelv studied at Pots- 



22. Cliffs of Potsdam Sandstone on the * . , 



Au Sable River, New York. dam and Iveeseville, 111 



