462 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



It is a dark-gray, almost apliauitic rock, with broad couchoidal fracture 

 and without auy tinge of red or brown in its color. It is faintly subpor- 

 phyritic, and with a lens the scattered, minute, squarish feldspar cross sections 

 appear, and at times a triclinic striation can be observed. At times, also, 

 one detects a black cleavage sui'face of augite, but only with difficulty. 



Under the microscope the rock is seen to be a typical diabase, the 

 network of elongate feldspars inclosing the shapeless masses of augite. 

 Two generations of feldspar, augite, magnetite, and apatite make up the 

 constituents. 



The larger feldspars of earlier generation are distantly scattered in 

 the field in squarish crystals or crystal groups, and measure about a mil- 

 limeter across, and this is by far the commonest size in all the slides I 

 have examined. They often show indication of transportation, being 

 broken, or showing undulatory extinction, or having an external band 

 which extinguishes differently from the central. A delicate zonal struc- 

 ture is at times present, or the center is full of opaque grains and the outer 

 portion limpid. These latter structure forms are more common in the dikes 

 than in the two large beds. The twinning striation is often interiiapted and 

 distant, so that quite broad patches belong to a single indi\'idual. At the 

 type locality these large crystals are exceptionally fresh for specimens out 

 of the large trap beds. They are, however, largely decomposed into a 

 mass of shapeless, brightly polarizing scales, apparently micaceous, while 

 more commonly both the generations of feldspar are decomjiosed into a 

 fibrous saussuritic mass. 



In a long series of observations of the extinction angle of porphyritic 

 crystals from every part of the valley, more than half the angles obtained 

 were about 31°. This would indicate strongly that the feldspar was 

 anorthite, which would agree with the results obtained by Mr. Hawes (cited 

 below, p. 464) in an analysis of the porphji'itic crystals of a dike cutting 

 West Rock in New Haven. I may recall, also, Hawes's suggestion that the 

 more difficult fusibility of anorthite may favor its earlier crystallization. 



The second generation of feldspar, which forms the latticework, is 

 lath-shaped, often with ragged ends and notched and irregular sides, and 

 averages 0.1™™ in length, though it is subject to more fluctuation than the 

 larger group. Its extinction angles vary from 12° to 26°, which would 

 best comport with the composition of labradorite. The rock under special 



