54 GEOLOGY OF THE NARRAGANSETT BASIN. 



decayed rocks. It happens that on the insland of Marthas Vmeyard tliere 

 are extensive deposits of arkose formed during the Tertiary period, which 

 ap})ears to indicate that materials of tliis nature may be transported for 

 considerable distances. The deposits at Gay Head contain very thick beds 

 of arkose, probably brought from the granitic area lying to the northwest 

 of the locality. It is difficult to conceive that the supply of the detritus 

 could have been brought from less than two score miles away from the 

 locahty in question. It is of course possible that these Tertiary arkoses 

 were derived from some granitic area near the place where they lie — an area 

 which by local subsidence or excessive erosion has in modern times been 

 brought below the level of the sea; but the evidence in these cases is clearlv 

 against this hypothesis and in favor of the assumption that the little-rounded 

 crystals from the granitic sources have been conveyed for forty or more 

 miles. Therefore, while the Carboiiiferous arkoses afford clear evidence 

 that they were deposited along the shores of a basin, no very clear evidence 

 as to the field of their derivation can be obtained from the conditions of 

 the beds. 



The arkose deposits of the Narragansett Basin are found in both 

 groups of deposits which have been observed in the field, the Cambrian 

 and the Carboniferous. In the former the evidence is limited to the region 

 about Attleboro, that being the only portion of the basin where the Cam- 

 brian strata are clearly recognizable. The Carboniferous arkoses are mucli 

 more extensively distributed. They may indeed be said to occur as a 

 characteristic feature in the margin of the basin, serving to show that, in a 

 general way at least, the trough existed with something like its present 

 horizontal limits as early as the time of the Coal Measures. At no other 

 point are the evidences as to the conditions under which the deposits were 

 laid down so clear, or at least so well ascertained, as at Steep Brook, but at 

 all points the facts show that the materials composing the beds have been 

 transported from a distance, and so far as determined the carriage has been 

 from the sides of the basin toward its central part. 



It is important to note that the arkose deposits of this district appear 

 to have been the first of a series of erosional phenomena leading in the 

 end to the formation of extensive conglomerates. The stages are, first, the 

 arkose material, or the waste of rocks decayed in place; next, the clays and 

 sands, which may be regarded as the product of ordinary erosion, wdien 



