ST. PETERS SANDSTONE. 287 



limestone clipping under it at an angle of 25, and rising thirty feet 

 above it, has been observed to be almost perfectly free from calca- 



reous matter. It is to be re- 

 marked, however, that the 

 arching form of these lime- 

 stone mounds, and the con- 

 I centric nature of the external 

 layers are most admirably 

 adapted to resist erosion, and 



SHOWING THE COLORATION OF ST. PETERS SANDSTONE that by the nearness of these 



domes to each other, they 



would afford mutual protection from violent wave action. Conglom- 

 eritic and brecciated rock, formed by fragments of the limestone im- 

 bedded in the sandstone, have been observed at several points, but 

 only in very close relation to the limestone. 



A much more prevalent modification, especially to the northward, 

 consists of thin seams of white argillaceous material interlaminated 

 with the sand and, to some extent, diffused through it, giving it a 

 schistose character. It then possesses considerable coherence, and is 

 found in large firm masses on the slope below the ledges of Trenton 

 limestone. At some points, ferruginous matter mingles with the cal- 

 careous and argillaceous ingredients, forming a variegated rock not 

 unlike the red and purple shales of the Mendota beds. 



In addition to the modifications arising from the Lower Magnesian 

 limestone, the Archaean rocks contribute others, which have perhaps 

 been sufficiently described in connection with the quartzites of Port- 

 land and "Waterloo. 



At several points in Hock county, the passage of the St. Peters to 

 the formation above is attended by an alternation of sandstone and 

 calcareous rock. The sandstone just below the calcareous bed is 

 marked with fucoidal impressions and the base of the calcareous layer 

 contains abundant Scolithus tubes. The calcareous bed is of a green- 

 ish gray cast containing a large percentage of insoluble, argillo-arena- 

 ceous material, in addition to the evident quartzose grains ihat are 

 more or less freely scattered through portions of it. This has not 

 been observed to attain a thickness of more than four or five feet. 

 The upper portion is usually shaly and appears at some points to 

 have been eroded before the deposition of the stratum of sandstone 

 above. This latter is thin and mixed with argillaceous material on 

 which sometimes supervenes a thin seam of carbonaceous matter fol- 

 lowed by the fossiliferous Trenton limestone. At the most northern 

 point at which the junction was seen, the sand mingles freely with 



