154 MINERALOGY AND LITHOLOGY." 
and which has gathered into little radial concretions in cavities, which 
are otherwise filled with calcite (see p. 120). Some of the augite in this 
rock is quite fresh, and some is entirely decomposed. 
Diabase (Porphyritic). All the remaining varieties of diabase that 
have been found in New Hampshire are porphyritic; and though many 
are massive, the development of large crystals in a ground mass of fine 
crystals is much more characteristic of the basic eruptive rocks of the 
state. Sometimes but one ingredient is porphyritically developed; and 
sometimes nearly all the constituents are in part large crystals. The 
ground mass is in no sense an amorphous or half crystalline substance, 
but is a fine-grained diabase; and therefore the difference between the 
massive and porphyritic varieties is merely a structural one, which is 
dependent on certain conditions which I shall endeavor to point out. 
The most common variety of porphyritic diabase is the one in which 
large crystals of labradorite are developed in a fine-grained ground mass. 
The rock is ordinarily called labradorite porphyry. With its large white 
crystals so conspicuous in their black surroundings, it is very beautiful. 
This feldspar is often perfectly fresh and undecomposed, and thin cleav- 
age pieces can be obtained, the optical properties of which prove the 
crystals to be labradorite. The angle between a plane of elasticity and 
the twinning plane, as measured in basal cleavage scales obtained from 
specimens from Ossipee and Center Harbor, is about seven degrees. 
The specimen from Ossipee will be described as typical. In thin sec- 
tions, the augite in the ground mass is seen to be altered into an aggre- 
gate of chlorite, calcite, etc., while the large and small crystals of labra- 
dorite are still intact. Two systems of twinning are often seen in the 
large crystals, which show very clear and distinct bands of color in 
polarized light. The other constituents and peculiarities of the rocks 
are those of common diabase, and which need not be repeated. Speci- 
mens from Center Harbor, and Concord, Vt., have been examined, and 
offer no further peculiarities. A specimen from Bartlett contains some 
quartz. In all the specimens the large crystals are flat and tabular, and 
hence on surfaces of fractures they appear long and narrow. This is 
because the lateral planes of the crystals are developed, but none others. 
The terminations of the crystals, as seen in the rocks, are consequently 
irregular. 
