MINERALOGY. 61 
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 Pl. 7, Fig. 2, which is drawn from 
the hornblende schist of Cornish. The plane of vibration of the light 
