r 



The Saint Peter Sandstone. 67 



From the fact that no Saint Peter sandstone has yet 

 been reported from the Upper Peninsula of Michigan, it has 

 been thought probable that a limit to the formation exists 

 in that direction, and one which existed necessarily before 

 the Trenton was deposited, since the Trenton is still there. 

 It is possible that failure to report is due to failure to 

 observe. 



The known limits of the Saint Peter sandstone are 

 those that have been produced by erosion, along the north- 

 ernly and more elevated extent of the formation; the 

 unknown limits are in the direction of the dip of the sedi- 

 mentary rocks, i. e., in a general southeast and southwest 

 direction. The Saint Peter may be continuous with forma- 

 tions of other adjacent areas but it has not yet been traced, 

 nor as it would seem, satisfactorily correlated. 



LITHOLOGIC CHARACTERS. 



The Saint Peter sandstone averages about 99 per cent, 

 silica in the form of grains of clear limpid quartz. There is 

 very little cementing material of any kind so that it forms a 



*'very friable stone so loosely united that it 



appears like sand" (Keating, 4). The grains are typically 

 clear white but the sand is often stained yellow or red 

 locally. It is rarely cemented into a firm rock by an 

 infiltration of iron oxide. Rarely also calcium carbonate 

 infiltration has produced large concretions and areas of firm 

 white rock. Normally it is easily eroded. 



The sand grains are so much worn that they are nearly 

 round and have a dead polish. Chamberlin (26) describes 

 them as irregularly angular grains like the quartz from 

 eroded acid eruptives and rarely of true crystalline form. 

 The writer has found quartz crystals ; but since the zones in 

 which they occur, cross sometimes the sedimentary lamina- 

 tion instead of coinciding with it they are to be considered 

 as originally rounded grains which have been rebuilt to 

 perfect crystals. The rounded grains are the typical ones. 

 In size, they vary from finest dust to coarse sand, 1 to 2 

 mm. in diameter, intermixed or imperfectly assorted- The 

 dazzling white color gives the appearance of greater uni- 

 formity in size of grain than really exists. 



