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University of California. 



[Vol. i. 



phane.* Under the microscope, a slide made from a fragment 

 taken immediately at the contact, shows a holocrystalline glauco- 

 phane schist, but of very fine texture, and traversed by numerous 

 veins of clear crystalline quartz. With a No. 8 Hartnack objective, 

 the glaucophane appears as small, slender, many-jointed prisms. 

 The large ones show the characteristic pleochroism, but this is 

 fainter in the more minute, and is not visible at all in the smallest 

 and thinnest individuals. Other minerals may be present with the 

 glaucophane, but, from the similarity of habit of all the prisms, this 

 is not very likely. These minute crystals are thickly crowded in a 

 fine mosaic of crystalline quartz, and possess a generally parallel 

 arrangement, so that aggregates of the prisms sometimes give the 

 pleochroism that is not perceptible in the single individuals. The 

 slide still shows traces of the original radiolaria in a mottled ap- 

 pearance, caused by the numerous spots of relatively clearer quartz. 

 The boundaries of these circular spots are shadowy, and merge into 

 the rest (if the groundmass. 



Similar evidences of alteration may be observed in the cherts 

 near Point Knox. A specimen from the central part of the chert 

 area, is a variegated jasper with splintery fracture. Under the micro- 

 scope it reveals a clear ground of quartz, ci ystallized into a fine 

 mosaic, through which are scattered abundant small crystals of 

 some mineral in slender-jointed prisms, with pointed and frayed 

 ends. The interference colors are fairly bright, and the index of 

 refraction high. The crystals are nearly colorless, but, when matted 

 into aggregates, are greenish yellow. Owing to their small size, it 

 is difficult to observe their extinction angles accurately. Although 

 most extinguish apparently parallel, some show small angles. The 

 mineral is probably a light-colored amphibole, but its determination 

 is not certain. It resembles some of the smaller microlites found 

 in the glaucophane schists. 



Near the fourchite this chert also assumes the blue tint indicative 

 of the presence of glaucophane. An inclusion in the eruptive on 



* Note. — It is quite p>ssible that some ot the acicular crystals of blue 

 amphibole occurring in the contact rocks are not truly glaucophane, but 

 some other soda amphibole. It is thought, however, that the use of the 

 name glaucophane , with this proviso, is preferable to the cumbersome rep- 

 etition of 6/ue amphibole in order to avoid a possible misnomer. 



