56 MINERALOGY AND LITHOLOGY. 
also a prominent ingredient in some of our most interesting rocks. In 
the town of Amherst, fine and much sought specimens, containing pyr- 
oxene, vesuvianite, and cinnamon garnets, associated together, are found 
in the limestone. Fine crystals are also found at Warren; and at Haver- 
hill there is a locality where beautiful green crystals of pyroxene are 
found associated, as at Amherst, with cinnamon garnets. 
Pyroxene is one of the most important minerals that plays a part as a 
rock constituent. It is most common in basic rocks, though it also en- 
ters at times as an essential into the more acidic rocks, as, for example, 
the augite sienites. Hence, the study of the mineral as a rock ingredient 
is most important in our state; and its constant recurrence in various 
forms renders this study interesting. 
In our rocks, augite occurs both in crystals and grains. When 
of sufficient size to be macroscopically examined, it is either black or 
dark in color, and the cleavage surfaces that it shows are at right angles 
to one another, so that it is without great difficulty distinguished from 
hornblende ; and when it sinks to smaller proportions, the microscope 
determines it with certainty. At times it is well crystallized in the 
rocks, as, for example, in the olivine diabase of Campton Falls. The 
outlines that are obtained in cutting a section of a rock where the augite 
is crystalline are such as would be obtained by cutting a crystal 
of the most ordinary and common form, for the rare and complicated 
forms that are found on free crystals do not occur on ingrown crystals. 
Fig. 3, on Pl. 4, is drawn from a section of the Campton Falls rock as 
an illustration, and a common augite crystal is introduced into the 
figure for comparison. It is seen that sections parallel to the base 
(section a) willbe eight-sided, those parallel to the orthopinnacoid (6) 
will be six-sided, while those parallel to the clinopinnacoid (c) will be 
four-sided, and modified and distorted figures will be obtained in oblique 
directions, but which can usually be identified. The cleavage is parallel 
to the faces of the prism 7; hence the basal sections show a right- 
angled cleavage, which is sometimes nearly perfect, as in this case, but 
more often it is interrupted, though it can almost always be recognized. 
The mineral being monoclinic, those sections that contain the ortho- 
diagonal (sections 4 and a) will be dark between crossed Nicols, when — 
a side of the prism is parallel to the plane of vibration of the light, while 
