24 GEOLOGY OP THE NARRAGANSETT BASIN. 



where to be observed in the New England troughs. In a measure, the dis- 

 tinct folding of the neighboring trough deposits on the east, occurring where 

 the basement beds are of an essentially nonschistose character, seems to 

 bear out this hypothesis. 



In the Marthas Vineyard area of distortions there is no indication of 

 a trough. There are no ancient rocks rising above the plane of the sea. It is, 

 however, quite possible that the northwestern border of the basin, if we 

 assume such to have existed, was in the bed rocks of the neighboring main- 

 land and that the seaward border has been worn away by marine action or 

 lies depressed below the sea level. It is to be noted that the prevailing 

 axes of the dislocations on Marthas Vineyard indicate a pressure acting 

 from the northeast and southwest, with resulting foldings which are mainly 

 aligned in a northwest-southeast direction, or approximately at right angles 

 to the usual trends of the Narragansett and other folds of this part of 

 the continent. Although it seems to me probable that these crumplings 

 of the Cretaceous and Tertiary beds of Marthas Vineyard were formed in 

 a trough which was filled in these ages, the evidence on this point is not 

 clear. So, too, with the dislocated deposits of Block Island. It will there- 

 fore be best to pass these areas by with the remark that the forms and 

 trends of their orographic reliefs differ widely from those of the older dis- 

 turbances, and that some of their peculiarities, especially the faultings and 

 complications of their folded strata, may be due to the fact that the move- 

 ments occurred near the surface, without, the restraint which is imposed on 

 beds, such as those in the Narragansett area, compressed while under a 

 thick mantle of deposits which have since been removed. 



In the Boston Basin we find series of rocks which have the same 

 general character as those in the Narragansett Basin. To a great extent, 

 the rocks, as regards their structure, are fairly comparable to those in the 

 Narragansett district. Although the evidence is not perfectly clear, it goes 

 to show that there was the condition of a preexisting basin, which was 

 formed sometime after the horizon of the pre-Cambrian, and which received 

 the deposits of the Roxbury conglomerate period, which make up the greater 

 part of the accumulations. As yet the age of these conglomerates and 

 the associated rocks is not determined, a most assiduous search having 

 failed to reveal fossils of determining value. Therefore it is not possible to 

 fix the earlier limit of the disturbances. Still, the immediate contact of the 



