32 CALIFORNIA ACADEMY OF SCIENCES. 



pretation of this mineral. It certainly is not biotite ; from 

 the form, habit, and other characteristics, I am very 

 strongly inclined to regard it as a somewhat abnormal 

 hornblende. 



Microlites, evidently very similar to these, have been 

 occasionally observed before,* and interpreted in the 

 same way; the rocks were hornblende-basalts, and the 

 microlites probably the result of a resorbing action of 

 the magma on hornblende crystals of an older generation. 

 It would be dithcult to prove the same with regard to 

 the rock in question, but it is at any rate certain that the 

 mineral belongs to one of the very first generation of 

 minerals. 



To the constituents named above is added a small 

 quantity of glass, wedged in between the feldspar crys- 

 tals. The rock is not porphyritic, that is, contains no re- 

 peated generations of the sam.e mineral; it is holocrystal- 

 line, with intersertal structure, and should be referred to 

 Meissen type of basalts. t 



It remains to describe the mica, coating the cavities of 

 the basalt. This mineral, detached from the rock and 

 mounted horizontally on a slide in Canada balsam ap- 

 pears as usually regular hexagonal foils of a light chest- 

 nut-brown color; between crossed nicols they do not 

 remain dark, but show, on revolving the table, dark-gray 

 or bluish colors, even when the crystal is of a minimal 

 thickness, and which in thicker foils go over into the 

 yellow colors of the first order. The pleochroismus is 

 quite strong on oP (ooi): Rays vibrating parallel to b, 

 or to ccPoo (oio) are yellowish-brown; these parallel 

 to c, or ccPco (loo) nearly colorless or slightly yellowish. 



*L. Van Werveke, Neues Jahrbuch, 1879, p. 824. H. Sommerlad, Neues Jahr- 

 buch, B. B., 1883, II, p. 139. 



tRosenbnsch, Mass. Gesteine 1887, p. 725. 



