CHAPTER X. 



THE ARKOSES AND BASAL CONGrLOMERATES. 



NATICK AKKOSE. 



From Natick to Cranston Aloilg tllG StGCp llUl faCO frOIXl Natickj Rliode Islaild, 



for 2|- miles northward into Cranston, extend the arkoses and qiiartzite con- 

 glomerates that form the lowest rocks of Carboniferous age on the western 

 side of Narragansett Basin. The eastward dip of these rocks shows their 

 position beneath the Kingstown series, which lie farther eastward. Near 

 the fork of the road, half a mile northwest of Natick, a coaly black shale 

 overlies the arkose and is itself overlain by a qnartzitic sandstone contain- 

 ing large qnartzite pebbles. The arkos^ consists largely of detrital quartz 

 derived from decayed granite, and is nsnally found near those localities 

 where the immediately underlying pre-Carboniferous rocks consist chiefly 

 of granite. The quartzite conglomerate is usually found near pre-Carbon- 

 iferous quartzite beds, and near contact with the latter the round pebbles 

 give way in places to those of such angular contours as to warrant giving 

 the name of breccias to the lowest layers. This is especially true of the 

 exposures at Alexander McTeer's house, north of Natick. Granite pebbles 

 are rare, owing to lack of firmness on the part of the granite in the original 

 ledges at the time the arkose was formed. Of course the arkose forms a 

 part of the detrital material between the pebbles of the coarse conglom- 

 erates, and quartzite pebbles are not rare in some of the beds of arkose. 

 In some cases the basal arkoses and quartzitic conglomerates do not exceed 

 100 feet in thickness, and a thickness of 200 feet seems not to be attained 

 in any single set of exposures. Some of the pebbles in these basal con- 

 glomerates are 15 inches in length. 



Northward, in the southern part of the field studied by Mr. Wood- 

 worth, the basal arkoses and conglomerates do not long continue to be 

 exposed, although the steep escarpment still outlines approximately the 

 position of these basal deposits. 



The basal arkose and conglomerate beds along the western border of 

 the Carboniferous basin are believed to represent the oldest Carboniferous 

 rocks in the areas bordering Narragansett Bay, They are not a formation 



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