302 S. IVEIDMAN 



connection with the origin of the clay, was unknown to Irving. 

 Indeed, the idea of peneplains or base-levels of erosion had 

 hardly been promulgated at the time of his writing in 1876. 

 While it is probable that the processes suggested by Irving may 

 have operated to some extent in the development of the clay 

 formation, it is not thought that either or both combined could 

 have been important factors. It is believed by the writer that 

 the wide-spread occurrence of the thick deposit of clay as a sub- 

 jacent formation of the Potsdam sandstone obviously points to its 

 origin and its presence there before the sandstone was deposited 

 upon it. This view is entirely in harmony with and strongly 

 supported by what has already been stated concerning the 

 geographic evidence of the peneplain character of the pre- 

 Cambrian land in pre-Potsdam time ; for if the pre-Cambrian 

 land surface were degraded to the near level of the sea, it is very 

 evident that the conditions for deep weathering and decom- 

 position of the surface would prevail over large portions of the 

 plain. In the process of degradation of the pre-Cambrian, the 

 less resistant rocks would first be cut down to the limit of slope 

 below which the streams could not longer transport the loose 

 material from the surface, and at this stage of the degradation, 

 when erosion would cease, the most favorable conditions for 

 chemical metamorphism would prevail, and the weathering and 

 decomposition of the rock into clay would be likely to extend 

 even to depths below the level of the sea. In fact, we could 

 hardly expect the degrading of a land area consisting mainly of 

 the silicate minerals like the pre-Cambrian rocks, to the near 

 level of the sea without considerable weathering of the flat-lying 

 surface. Hence the position of the clay beneath the sandstone, 

 in view of the peneplain character of the pre-Cambrian land in 

 pre-Potsdam time, obviously points, it is believed, to its develop- 

 ment there before the deposition of the Potsdam formation. 



Distributioji of Potsdam co7iglomerate . — About the isolated 

 sandstone outliers upon the slightly dissected peneplain, and 

 along the crystalline rapids in the border of the sandstone dis- 

 trict there is a general absence of conglomerate, or even a 

 semblance of coarse sediment, at the base of the Potsdam sand- 



