396 A. C. LANE 



ORDOVICIAN 



II. St. Peter sandstone. — There is no probability that these 

 periods are of uniform length or intensity of oscillation. The 

 unsteady period between the Trenton and Calciferous (Canadic- 

 Ordovicic) appears to be much less important than the great period 

 terminating in the Lake Superior overlap which in Michigan at least 

 may include a good part of Ulrich's Ozarkian/ and perhaps earlier 

 (the Keweenawan) during which Michigan was out of water and 

 rent by tremendous volcanic outbursts. 



In an emergence sandstone the sand itself, being rehandled along 

 a rising coast, is more likely to be uniform in texture. Its connate 

 water may be fresh at the margin. In many respects the St. Peter 

 sandstone is a typical emergence sandstone as compared with the 

 Lake Superior sandstone and that is one reason why I do not think 

 it extends into the Lake Superior basin. As Schuchert's curve^ 

 indicates it does not mark so much emergence. My remarks in 

 Part I that the viewpoint of one studying drillings is different from 

 that of one studying outcrops in such a basin as Lower Michigan 

 must not be forgotten. The gaps due to discordances and discon- 

 formities are liable to be much greater; the emergence sandstones 

 less at the outcrop that at the center of the basin. Both the St. Peter 

 and the Berea I take to be emergence sandstones in Michigan, and 

 neither of them have been recognized as outcrops, though distinct 

 in some drill records. The base of an emergence sandstone is prob- 

 ably a more definite datum plane than that of a submergence sand- 

 stone. 



The greatest thickness of the St. Peter seems to be to the south- 

 west. Just outside the state at Marinette, No. 2 well apparently 

 gives 75 feet of it from 325-400 feet. But how rapidly it thins and 

 how irregular it is, is shown by the fact that at Gladstone it was not 

 distinctly recognized, and across the Bay de Noe perhaps represented 

 only by a red clay shale, the weathered surface of the Calciferous dolo- 

 mite. It seems to fill hollows in the eroded Calciferous quite as in 

 Wisconsin. Farther east it is not known. The Pickford record is 

 imperfect, and in the Neebish samples, if present, it is indistin- 

 guishable from the Lake Superior sandstone. 



1 Schuchert, Science, XXIX (1909), 630; Bull. G.S.A., XX, No. 20. 



2 Op. cit., PI. loi. 



