138 GEOLOGY OP THE NARRAGANSETT BASIN. 



so far as it was a lowland or valley, as Professor Shaler supposes it to 

 have been. That a long period of erosion or nondeposition had closed at 

 this time is abundantly proved by the marked unconformity of the Carbon- 

 iferous with the underlying terranes, this great stratigraphic break embrac- 

 ing, as above noted, all of Silurian and Devonian time, if not as well the 

 upper Cambrian at the beginning of this interval and the lower Carbonifer- 

 ous at the close. 



Absence of iron oxides in the basai arkose. — The prevailing whitish and grayish hues 

 of the basal arkoses, and of the conglomerates immediately above them, are 

 in strong contrast to the often vivid reds of the overlying and occasionally 

 intercalated Wamsutta series to be described beyond. This bleaching of 

 the first Carboniferous sediments appears, like the coloring of the Wamsutta 

 beds, to have taken place prior to transportation, and the two processes 

 may be said to be complementary to each other. The granitites along the 

 northern border, and almost everywhere adjacent to the portions of the 

 basin where the red series of strata occur, are strikingly red by reason of 

 the discoloration of one of their feldspars. This reddening is probably one 

 of the results of alteration through atmospheric decay, but it is difficult to 

 determine when this change in the feldspar began to take place in an effec- 

 tive way, for the granitites in the present superficial zone have at least twice 

 been exposed to meteoric waters, once at the beginning of the Carboniferous 

 deposition, when the shore line was creeping over the rock, and since then 

 when the Carboniferous covering was swept away and the rocks were again 

 bared to weathering. It is hardly possible to suppose that the decay which 

 has led to this reddening occurred in Carboniferous time, for the reason that 

 the pebbles of granitite found in the conglomerates of the red Wamsutta 

 series, or higher up, are, so far as I have observed them, never red except by 

 absorption of the red paste in which they are embedded. The most reason- 

 able supposition which I am able to advance is that the superficial products 

 of weathering previous to their transportation in Carboniferous time were 

 leached of their iron salts, which penetrated downward. The first transpor- 

 tation of detritus affected the superficial leached layer, and thus the basal 

 beds came to be white, as we now see them. When erosion had stripped 

 away the bleached materials at the surface, it reached the highly discolored 

 rock beneath, as yet very imperfectly disintegrated, from which were pro- 

 duced the red beds of the Wamsutta series. 



