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GEOLOGY OF THE NAKRAGANSETT BASIN. 



The felsites frequently occur higher up stratigraphically than the 

 intruded diabases. The following scheme of arrangement of rocks at three 

 localities will represent this fact: 



Stratigraphic relations offelsite and diabase at three localities. 



This matching of short sections within 2 or 3 miles of each other, the 

 first two being within half a mile, illustrates something of the constancy of 

 occurrence of these igneous rocks. Regarded as a map, the bottom of the 

 table is east, the top west. The interrogation marks indicate the places of 

 concealed strata. The persistent failure of the western contact of the felsite 

 is a noticeable feature, due to the erosion and concealment of softer material. 



The large felsite mass between the village of South Attleboro and Red 

 Rock Hill causes the strata to separate in the manner of a tilted laccolith, 

 but contacts have not been observed which verify the view that it is one. 

 Flow structure, often attended with crumpling of the layers, is manifest in 

 many outcrops of this rock. 



Beneath the massive flow of the felsite is a zone of the same rock, form- 

 ing the matrix of an agglomerate, composed of rounded pebbles of felsite 

 and quartz-porphyry, together with quartzite and occasional pieces of horn- 

 blendic granitite. This lower bed is several feet thick. In South Attleboro 

 there is exposed a bed having a thickness of more than 10 feet. The water- 

 worn pebbles are evidently fragments caught up in a movement or flow of 

 the felsite over earlier conglomerates. The groundmass of this agglomerate 

 is porphyritic, with a plagioclase feldspar in every respect like that of the 

 overlying mass. 



