MINERALOGY. 6 1 



Tremolite is the white variety containing no iron. It is noticeable at 

 Bedford (near the Devil's Den, abundant), Gilmanton, and Warren. 



When this variety of amphibole crystallizes in fine capillary form, 

 it is called asbestus. This variety is found at Franconia in masses or 

 sheets which are from one to two inches thick, and composed of the 

 finest interwoven fibres. This is called mountain leather. It is notice- 

 able, also, on Monadnock mountain. A fibrous, dark colored variety, 

 resembling fossil wood, is found at Lebanon. 



Hornblende is a most important mineral as an ingredient of the 

 rocks. In combination with feldspar, it forms our sienites, and with 

 a triclinic feldspar or with quartz, it forms that wide expanse of 

 diorites and amphibolites that occupies so much of the Connecticut 

 valley. It is also a prominent ingredient of the eruptive rocks. It 

 is common in works on lithology to divide it into two kinds, — basaltic 

 and common hornblende. Basaltic hornblende is that very deep colored 

 ferruginous hornblende that occurs in the basic eruptive rocks. The 

 sections must be made thin, in order to make it transparent. It is usu- 

 ally deep brown, and strongly dichroic. Such is the hornblende of the 

 eruptive diorites at Campton falls, Dixville Notch, etc. The common 

 hornblende is lighter in color, contains less iron, is more often green, and 

 is not in such compact crystals, being very often in fibrous masses or 

 crystals made up of numerous others. It is more or less dichroic, accord- 

 ing to the depth of its color. Such varieties as actinolite, which in thin 

 sections become white, of course are not dichroic. 



Hornblende is most easily recognized by its cleavage, which is so 

 perfect, parallel to the sides of its first prism, that all basal sections 

 appear divided up into rhombs with an obtuse angle of 124°. This 

 characteristic serves for the determination of hornblende in all cases, 

 save in those in which it exists in aggregations of minute crystals too 

 small to exhibit cleavage, as it often does. 



The pleochroism of hornblende is so remarkable that it aids in its 

 determination. Hornblende is monoclinic, and hence it is possible that 

 the light traversing the crystal parallel to its three varying planes of 

 elasticity may be differently colored, and this is markedly the case with 

 this mineral. This is illustrated in PI. 7, Fig. 2, which is drawn from 

 the hornblende schist of Cornish. The plane of vibration of the light 



