206 



University of California. 



[Vol. r. 



by Williams, in which he says, "The trap (fourchite) shows consider- 

 able variation in different parts, both in its texture and in its miner- 

 alogic constituents, so that, petrog-raphically considered, it should 

 be classed under several heads."* 



A very noticeable feature in these sections of the fourchite is 

 the presence of a bright blue amphibole, having the pleochroism of 

 glaucophane, which appears to have been developed secondarily, 

 parti}' at the expense of the augite. It is often in the merest films, 

 occupying interstitial spaces, or filling crack's in the grains of augite. 

 With very high powers, however, these films can generally be seen 

 to be made up of aggregates of extremely delicate needles, having 

 a generally parallel arrangement. At other places portions of the 

 augite grains have been wholly replaced by the blue amphibole. 

 In such cases the fibers or needles of amphibole appear to be gen- 

 erally parallel with the prismatic axis of the augite. The alteration 

 differs from ordinary uralitization in being comparatively superficial. 

 The greater part of a grain of augite will be perfectly fresh and 

 unaltered, while the amphibole bounds it peripherally, occupies 

 some corner, or extends into the cracks in the crystal. Moreover, 

 the resulting hornblende is not confined within the original bound- 

 aries of the augite, but spreads out into the interstitial spaces 

 between the grains, and even forms small, though well defined, 

 prisms, with somewhat fibrous structure and ragged terminations, 

 which are not in direct contact with any augite. In general, however, 

 it forms irregular filmy areas, which only under high power show 

 a parallel fibrous arrangement. The most common pleochroism is 

 from a brilliant ultramarine blue to a rich violet. Less frequently 

 a light yellowish-green color is observed. The extinction angle is 

 small, certainly less than ten degrees. From its similarity to some 

 of the glaucophane of the contact metamorphic rocks, it is probably 

 the same mineral, but ma)' quite possibly belong to some closely 

 related member of the blue amphibole series. It does not appear 

 to have originated wholly from the augite, although it often replaces 

 the latter mineral. Its formation was probably contemporaneous 

 with that of the zoisite prisms, and it may have been formed in part 

 from the same unknown mineral as the latter. 



-Igneous Rocks of Arkansas, Ann. Rept. Geol. Surv. of Arkansas for 1890, 

 Vol. II, p. 30. 



