28 GEOLOGY OF THE NARRAGANSETT BASIN. 



So far as has been observed, all of the numerous intrusions occur on 

 the marginal portions of the basin, mainly on its western side and in the 

 prolongation of the area in what is known as Norfolk Basin, a field which, 

 as elsewhere noted, is not much considered in this report. The eastern 

 margin of the area is not so well revealed as the western, but, as will be 

 seen in the detailed descriptions of Messrs. Foerste and Woodworth, with 

 the exception of the felsite dike in Plympton, no intrusive masses have been 

 discovered on this margin. Dikes also occur, as indicated in Dr. Foerste's 

 reports, on the southern portion of the field, but not so abundantly as on 

 the western versant. So far as the observations go, they make it improb- 

 able that, in general, any dike attains the surface at a point moi'e than 2 

 miles toward the interior from the border of the Carboniferous field. An 

 exception to this statement must be made in the case of the Wamsutta field, 

 where, perhaps owing to the large amount of disturbance the beds have 

 undergone, dikes are found at a distance of nearly 4 miles from the western 

 border. 



It may also be remarked that the extended study of the rocks in the 

 central portions of the area has shown that, while dikes may perhaps have 

 penetrated to the lower parts of the section, there is no evidence afforded 

 by the bare rock surfaces, or by the materials of the drift so far as observed,- 

 which would lead to the supposition that these injections penetrated into 

 the zone of the upper conglomerates. 



Perhaps the most interesting group of what appear to be intrusive 

 masses is that of the pegmatites which occur in the southern portion of the 

 western margin. As we go southward from Providence there is a gradual 

 increase in the measure of metamorphism to which the Carboniferous 

 strata have been exposed. The observer is led to suspect the existence of 

 some extensive concealed intrusion which has applied much heat to the 

 section. These indications of metamorphism increase until they attain their 

 maximum in the portions of the field in and about Boston Neck and Tower 

 Hill. Where the alteration of the strata is most considerable — where, 

 indeed, those beds appear as ordinary gneisses — we find extensive pegma- 

 tite intrusions, which penetrate these conglomerate and sandstone gneisses. 

 1 have been unable to determine whether these intrusions are to be classed 

 as dikes or as veins. So far as observed, the facts hardly warrant the 

 assumption that the metamorphism is directly due to the incoming of the 



