164 MINERALOGY AND LITHOLOGY. 



Plagioclase Diorite. ( PorpJiyritic Diorite.) There is another kind of 

 diorite, which has been found in boulders scattered all over New Hamp- 

 shire, and which is in place in the Dixville Notch and at other points in 

 the northern part of the state. This diorite is characterized by the por- 

 phyritic development of all its ingredients ; but large grains, which are 

 sometimes an inch and a half in diameter, of a clear white, glassy feld- 

 spar, are particularly conspicuous. This feldspar is in condition for 

 accurate determination, and proves to be a variety of plagioclase; and 

 hence this diorite can be more definitely classified than the others thus 

 far described. The feldspar is very striking in appearance. At the first 

 glance it looks like quartz, for, in some directions, its fracture is vitreous, 

 but on examining further, in other directions, bright cleavage faces are 

 identified. In composition, it is near andesite, and its analysis has been 

 given under that head (see p. 96). In polarized light few bands are 

 found, for its separate laminse are quite broad. Its optical properties 

 are those of oligoclase, and, according to Des Cloizeaux, andesite is 

 identical with oligoclase. I call the diorite, plagioclase diorite, because 

 a variation no greater than what is very liable to occur would make this 

 feldspar labradorite or oligoclase ; and the members of the class of plagio- 

 clase diorites are subject to variations which embrace this sub-species. 

 The hornblende of this rock was also analyzed. It was found to be 

 quite aluminous. The magnetite is also porphyritically developed ; and 

 even the apatite is macroscopically visible in long, slender, clear needles, 

 which pierce indiscriminately through all the other ingredients. The 

 ground mass of this rock is an aggregate of the same constituent. The 

 strong contrasts between the bright black and clear white crystals, and 

 the dark, compact ground mass, make this one of the most striking rocks 

 that occur in our state. 



When microscopically examined, this andesite is found to be fresh 

 and clear, and in polarized light shows no effect of decomposition. In 

 this respect it is almost isolated among our basic feldspars, and furnishes 

 a good illustration of the greater power of a glassy mineral to resist 

 decay; but in a specimen of this rock from Dorchester, in which the crys- 

 tals are very large, all the minerals arc much altered, and epidote becomes 

 a prominent ingredient of the rock. 



Another diorite at Dixville is worthy of mention. This rock cuts the 



