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 ( PorpJiyritic). 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. 



