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University of California . 



LV'OL. I. 



short lath-shaped, with the ends either squarely truncated or 

 rounded. The form of the cross sections could not be made out 

 with certainty, but some were apparently square. The index of 

 refraction is high, and the relief considerable. The double refrac- 

 tion is in general low, the interference colors being gray or yellow, 

 but varying somewhat, and becoming more brilliant in certain slides. 

 The extinction appeared to be strictly parallel with the sides of the 

 prism, and the largest individuals gave an interference figure, which, 

 while indistinct on account of their small size, appeared to be 

 unmistakably biaxial. The mineral therefore probably belongs to 

 the orthorhombic system. By using the quartz wedge it was 

 ascertained that the prismatic axis could not be the axis of least 

 elasticity. Cleavage was not detected with absolute certainty, yet 

 by careful focusing, with high powers, it is possible to distinguish 

 a breaking up of the light, as if from very fine cracks, and in one 

 or two cases extremely fine lines parallel with the prismatic axis 

 appeared to be visible. There is exhibited, also, an irregular trans- 

 verse parting in many of the prisms. The crystals showed no 

 signs of gelatinization when treated with warm hydrochloric acid. 



The secondary nature of this mineral was, by the study of cer- 

 tain peripheral fades of the fourchite, placed beyond reasonable 

 doubt. Certain local, and more or less glassy, variations of the 

 fourchite contain large phenocrysts of plagioclase, often corroded 

 and irregular in outline. When the original feldspar still remains, 

 these porphyritic crystals show the usual twinning lamellae of 

 plagioclase, but in by far the greater number of cases they have 

 changed into an aggregate composed mainly of the prismatic min- 

 eral just described, with small amounts of a greenish amorphous 

 substance. In some slides these phenocrysts are so irregular in 

 outline, and so completely altered to aggregates, that their original 

 feldspathic character would have remained extremely doubtful had 

 not other sections shown individuals which revealed at the same 

 time remnants of the plagioclase twinning lamellae and the devel- 

 opment of very minute prismatic microlites. It thus appears that 

 the small crystals occurring so abundantly in the groundmass of 

 the fourchite may, in certain cases, originate from the breaking 

 down of a plagioclase feldspar, and are presumably secondary, in 



