DIABASE DIKES AND STOCKS IN THE GNEISS. 413 



The above <likes near Millers Falls are of ideal freshness; very rarely 

 one sees in a single large feldspar a slight central clouding, like a delicate 

 fleck of cotton. They are rather light-gray, extremely tough, and yet l)rittle 

 as glass. The constituents are of exactly the averages dimensions given in 

 the general description of the diabase, page 438. The augite is yellow to 

 amethystine, dichroic, and, although perfectly fresh, it appears, from the 

 strong cleavage and abundant inclusions, only translucent, and looks in the 

 slide as if a quantity of pulverized material had been spread over the network 

 of feldspars. This enables one to distinguish it from other occurrences. 



5. Across Montague and Leverett no other outcrops occur. In Pelham, 

 on the south side of the Shutesbury road, west of where it crosses Ametln-st 

 Brook, a great outcrop of the same fine-grained diabase occurs in the actin- 

 olitic quartzite. It is a squarish mass about 82 by 130 feet, its lonoest 

 diameter north and south. Following the stream up from this point to where 

 a brook comes in from the north, one finds a great number of large bowlders 

 of diabase in its bed, some of large size. There is probably a considerable 

 bed in tlie pasture a few rods east of the junction of the brooks. Further, 

 the fine amethysts which occur as rolled specimens in the bed of the brook 

 ])robal)ly indicate the presence of diabase here. A mile northeast also, in 

 the deep brook gorge north of Ward's quarry, occur a great number of 

 very large diabase bowlders, as well as much farther east in the eastern 

 portion of the town, along the roads that run down from Pelham Center 

 to the Swift River Valley. 



6. If the line connecting the above outcrops in Pelham be prolonged 

 N. 40° E. into Shutesbury, it strikes a great outcrop of diabase at the point 

 where it crosses the road going north from Pelham Center, opposite the 

 house of W. Thrasher. It is exposed with a length of 2.5 rods and a width 

 of 75 feet, and runs N. 40° E. It is a fine to very fine, ver}^ fresh diabase 

 of the common structure, the finest-grained portion showing a globiditic 

 groundmass as inclusion in the feldspars, and small olivines. 



7. If the line be prolonged N. 40° E. across to the river road, another 

 outcrop of diabase occurs on the hillside northwest of the house of S. H. 

 Stowell. An inspection of the map will show that all the above series of 

 outcrops occur along a northeast fault which has opened the entrance to 

 the upland basin of Pelham and caused the sharp southern slope of Mount 

 Hygeia. 



