66 MINERALOGY AND LITHOLOGY. 



Silica, 49-03 



Alumina, 13-72 



Iron protoxide, ..... .... 9.84 



Manganese protoxide, .40 



Lime, 11.22 



Magnesia, 11.96 



Soda, ........ ... 2.40 



Water, ... .90 



99-47 

 This is the variety of hornblende that is called pargasite, and which 

 oft-repeated analyses have shown to be the common hornblende of green 

 diorites, in all localities. This hornblende, the analysis of which is given, 

 is a foliated variety, and by some is supposed to be a mixture of horn- 

 blende and pyroxene. In our rocks, at least, it is all hornblende, and its 

 aluminous nature is what I wish to call attention to in this connection. 



Hornblende is subject to decomposition, perhaps not so readily as 

 augite, but yet in an analogous way. In the rocks it is liable to be hydra- 

 ted, and to be changed into chlorite, and sometimes it breaks up into a 

 variety of products at once ; and although the external form remains, mi- 

 croscopic sections show that it is composed within of the most heterogen- 

 eous mixture. Fig. 3, on PI. 7, represents such a crystal drawn from a 

 section of the diorite which forms a dyke near the Profile house, Franconia. 

 It has the form of a quite perfect crystal of hornblende, but now it is com- 

 posed of magnetite, biotite, and calcite, with remnants of hornblende. In 

 many of the crystals in this rock the alteration has been complete. I am 

 inclined to think as Zirkel does, in his consideration of analogous augite 

 crystals, that the biotite was an enclosure, as calcite encloses sand, for 

 crystals, apparently undecomposed, are often penetrated by it ; but the 

 magnetite and calcite are products of decomposition. 



45. Beryl [Bcs AI2 Sig 0,s]. 



The largest beryls of the world are found in New Hampshire ; indeed, 

 our beryls are cited in every text-book to illustrate to what proportions 

 crystals can grow under favoring circumstances. Grafton and Acworth 

 are the most celebrated localities for great beryls. It is a hexagonal 

 mineral, and some of the large crystals have very perfect hexagonal forms, 

 though they lack the lustre and transparency possessed by the small 



