152 GEOLOGY OF THE NARBAGANSETT BASIN. 



it is a member of the Wamsutta series. Fossils have not been found from 

 which to determine the age of the beds independently. 



The texture and color of the rock, as well as its position and quantity, 

 would make it fit for building stone but for the fact that it is quite devoid 

 of those sets of joints or bedding planes on which the extraction of suitable 

 blocks depends. 



IGNEOUS ASSOCIATES OF THE WAMSUTTA GROUP. 



One of the striking features of the Narragansett Basin is the localiza- 

 tion of eruptive rocks in the area of the red strata of the Wamsutta group. 

 Dikes occur, however, elsewhere in this region, in Lincoln, near Providence, 

 and at the mouth of Narragansett Bay, marginal to the field. 



Diabase. — An interrupted faulted series of narrow, partly altered diabase 

 dikes can be traced from North Attleboro southward around the horseshoe 

 fold of the Wamsutta group to Lanesville and thence northward toward 

 Arnolds Mills. The diabase is usually erupted through red conglomerate, 

 sometimes in the form of twin dikes with a large sliver or wedge of the 

 country rock between. The upper surface of the diabase for a thickness ot 

 from 1 to 3 or even more feet is commonly vesicular; sometimes the lower 

 surface is amygdalar; but there is no evidence to show that the diabase 

 flowed out as a contemporaneous sheet. 



These dikes are of variable widths from point to point where they 

 appear, attaining thicknesses of from 20 to 50 feet. They frequently rise up 

 as low black knobs, as between North Attleboro and Attleboro Falls, or 

 appear as low bluffs, as on the east bank of Abbots Run and between 

 Adamsdale and South Attleboro. For the most part they crop out along 

 the outer limits of the circular area occupied by the red rocks. 



At a number of points these diabase knobs are so situated as to be 

 available for supplies of road stone, for which purpose they are superior to 

 any other rock in this district. The outcrops at Attleboro Falls are within 

 sight of the railroad, and there is a mass adequate for local uses free above 

 ground and now a hindrance to house building. 



The ledge on the Henry Guild place in Adamsdale and its continuation 

 northward affords another source of supply— the nearest locality of trap in 

 workable quantity to the cities of Pawtucket and Providence. It would 

 require a carriage of a mile to place the material on the cars of the New 

 York, New Haven and Hartford Railroad at Adamsdale Station. To run a 



