WAMSUTTA GROUP. 155 



The eruptions of felsite in this field appear to have taken place some- 

 time after the deposition of the first sediments of the Carboniferous section 

 and before the laying down of the Coal Measures along- the northern 

 border. These members of the quartz-porpyhry family of igneous rocks are 

 but outliers of more extensive eruptions of a closely related magma which 

 is extensively intruded into the rocks of the Boston Basin or is found there 

 as ancient flows. In that area the age of the eruptions is not precisely 

 known. If the evidence from the area of the Wamsutta group in North 

 Attleboro and the case in Plympton can be relied upon as evidence, it 

 would point to the Carboniferous age of these eruptives in the vicinity 

 of Boston, and probably to an epoch later than the lower Carboniferous 

 proper. 



DIAMOND HILL QUARTZ MASS. 



Lying on the western border of the Wamsutta group, but apparently 

 developed in these Carboniferous sediments and in the felsites, is the large 

 mass of vein quartz known as Diamond Hill. The quartz occurs prevail- 

 ingly in the vein form, with layer upon layer of divergent pyramidal-faced 

 crystals. Locally the quartz is chalcedonic and white, earthy, opaline, the 

 whole being evidently the product of hot springs following the decadence 

 of igneous action in this area. 



Quartz veins having the same structure and habit penetrate the red 

 sandstones of the Wamsutta group along- the northern boundary in Wren- 

 tham. This habit of crystallization has not been detected elsewhere in the 

 basin, although extensive quartz masses occur at other points, as at Mount 

 Hope, and in less abundance southward in the bay region. It is highly 

 probable that the deposition of this quartz took place during Wamsutta 

 time. 



WAMSUTTA VOLCANOES. 



The peculiar features of the Wamsutta series — the rapid thickening of 

 the sandstones and conglomerates toward the northwest corner of the present 

 area, the felsites with definite flow structure, the gray ash beds or Attleboro 

 sandstone, the agglomerates of felsitic material, and the associated conglomer- 

 ates composed in large part of felsite pebbles — all point to a volcano or volca- 

 noes existing in this field in Carboniferous time. The known petrographic 

 connection between the flow structure of felsites in extrusive masses and the 

 coarser structure of typical granite-porphvries in stocks and dikes brings 



