154 



University of California. 



IVOL. 3. 



to which the obscure pericline striation was slightly inclined. 

 The indistinctness of the twin lamellae is highly characteristic of 

 anorthoclase, and the small value of the angle V is a character 

 distinguishing that species from all other triclinic feldspars. 

 There is no doubt, therefore, that the dominant feldspar of these 

 rhyolites is anorthoclase. 



The structure of the groundmass is in some ways so unusual 

 that it will require a rather extended description. Two slightly 

 different varieties may be recognized, and will be considered 

 separately. 



In one of these the groundmass, by ordinary light, has an 

 appearance similar to that of some glassy rhyolites with well 

 defined flow- structures. Separated by a subordinate amount of 

 cloudy substance are clear brown, sinuous areas, sometimes 

 branching, which have a general tendency to extend themselves 

 in a common direction and curve about the phenocyrsts like lines 

 of flow, which they doubtless represent. But on close exami- 

 nation, and especially in polarized light, each of these streaks is 

 seen to be made up of two rows, separated by a sharp central 

 line of short fibres standing normal to the edges. The small 

 size of these fibres is an obstacle to optical investigation, but 

 they are found to have a weak double refraction and apparently 

 a rather large angle of extinction. The material between the 

 fibrous streaks is generally cloudy and cryptocrystalline, appear- 

 ing brightly speckled between crossed nicols. Sometimes, how- 

 ever, there is a little apparently amorphous, clear, light brown 

 material, which shows between crossed nicols cloudy brushes 

 parallel to the ctoss wires. A more typical exemplification of 

 this phenomenon is found in a rock soon to be described, and 

 it will not be discussed at present. Grains of quartz large 

 enough to be determined as such are sprinkled rather abundantly 

 through the groundmass, always lying between and not in the 

 sinuous, fibrous bands. 



Occurring locally are almond-shaped bodies, sometimes several 

 millimetres in length, often distinguishable from the groundmass 

 macroscopically by their lighter gray color. They are found to 

 be made up of rather large fibres, grouped in slightly divergent 

 bundles, suggesting a half-open fan. The fibres are colorless 



