126 GEOLOGY OF THE NABBAGANSETT BASIN. 



The angular fragments of the pre-Carboniferous rocks contained in these 

 beds, together with the meridional strikes of the Carboniferous as con- 

 trasted with the nearly east-west strikes of the older clastic series, afford 

 abundant evidence of the unconformity. The exact contact is not shown. 

 North of this locality the Carboniferous beds do not appear clinging to the 

 escarpment above the level of the glacial sand plains. In succession along 

 this escarpment, granite, gneiss, schists, and quartzite come up to the plane 

 of the base of the Carboniferous, indicating the varied lithological character 

 and structure of the floor on which the sediments were laid down along 

 this western border. Direct evidence of faulting is wanting. For most of 

 the distance the lowest and nearest visible outcrops of the Carboniferous 

 are from 2,000 to 3,000 feet eastward of the line, and probably at approxi- 

 mately that distance above the base of the series. Throughout this section 

 a distinct valley exists along the contact; its western wall is formed by the 

 escarpment of pre-Carboniferous rocks, including some Carboniferous beds 

 lying to the west and forming a part of the escarpment at the point of 

 beginning; 1 its eastern wall is formed by a broken ridge of hard sandstones 

 which stand up to the level of the adjacent peneplain developed on the 

 crystalline area to the westward. This ridge is broken through at Cranston 

 and Olneyville; and in each case the gap is opposite a valley opening 

 eastward out of the crystalline area. The deep reentrants in the pre- 

 Carboniferous rocks are thus shown to be of a date later than the Carbon- 

 iferous period and in no way affect the boundary line by their having 

 been originally filled with sediments. On the assumption that the peneplain 

 is of Jura-Cretaceous age, the denudation of the Carboniferous soft rocks 

 below that level is an index of Eocene and later erosion, and these valleys 

 along and across the contact are post-Cretaceous. The cross valleys are 

 not of glacial origin; the movement of the ice was nearly at right angles 

 to their course. The same remarks concerning the age of topographic 

 features apply to the valley of the Blackstone, the course of which is else- 

 where described in this report. It suffices to state here that it turns from 

 a southeast to a south course immediately on entering the basin. 



'In tracing the boundaries of the rocks lying on the western boundary of the Narragansett 

 Basin, Mr. J. H. Perry, of the United States Geological Survey, has recently shown that metamorphic 

 Carboniferous arkoses and cons'lomerates occupy a small area, about half a mile wide, lying to the 

 west of the boundary as drawn by Dr. Foerste and myself where our maps join. 



