96 MINERALOGY AND LITHOLOGY. 



59. Andesite [(Ca, Na'^) Al^ vSi^ O'^]. 



This is the feldspar intermediate in composition between labradorite 

 and oligoclase. The necessity of its recognition as a species is question- 

 able, since, chemically considered, the lines of its division from the two 

 species mentioned are indistinct, and, optically, it cannot be distinguished 

 from oligoclase. 



A feldspar occurs in the Dixville Notch, which is very noticeable on 

 account of its very clear, glassy lustre, and its perfectly undecomposed 

 appearance. It is found in the diorite, which by its decay has formed 

 a bed through which a stream runs, and forms the Twin cascade, — 

 one of our most beautiful falls. It gives the rock a remarkable appear- 

 ance, for it is found in rounded pieces as large as walnuts, and the rock, 

 when broken open, exhibiting these large, round, glassy spots, and large, 

 black hornblende crystals, is quite striking in appearance. It would 

 make a beautiful stone, if polished. This feldspar has a cleavage in two 

 directions, as usual, though it often shows conchoidal fractures like quartz, 

 which are deceptive. The crystals are twinned, but the separate laminae 

 are broad, and there is, consequently, not such a fine striation as is com- 

 mon in these species. My analysis of this feldspar from Dixville is as 

 follows : 



Silica, 56.24 



Alumina, 26.95 



Iron sesquioxide, tr. 



Lime 9-37 



Soda 4-93 



Potash, 1.25 



Water, • 1.15 



99.89 



This analysis gives as a quantivalent ratio, 1:3:7, which shows it to be 

 one of those varieties intermediate in composition between labradorite 

 and oligoclase, and may be called andesite. A basal cleavage piece 

 shows that an elasticity axis makes an angle of four degrees with the 

 edge of the brachy-pinnacoid, which is an angle to which oligoclase some- 

 times reaches. Its sections show that it is pierced with a few large 

 apatite needles, not enough, however, to influence the analysis. 



