]68 



University of California. 



1 Vol. 3. 



The lava which usually underlies this tuff is evidently of the 

 same chemical character. We may abridge its description by 

 referring back to the gray glass rhyolite of the Turtle Cove, to 

 which this rock bears a striking similarity in general appearance, 

 structure of groundmass, and character of phenocrysts. These 

 last are mainly anorthoclase, as proved by the shadowy twin 

 striation, small axial angle, and the characteristic extinction 

 angle measured in several sections normal to the obtuse bisectrix. 

 One nearly uniaxial sanidine crystal was found. An occasional 

 corroded phenocryst of quartz is seen. The groundmass is not 

 clouded as in the Cove rhyolite, but the glass, appearing clear 

 and homogeneous in ordinary light, shows the same shadowy 

 brushes when between crossed nicols. 



ROCKS OP RECENT FORMATION. 



This material is friable and very fine-grained and of a cream- 

 white color. All but a very small portion is made up of glass 

 fragments which are angular and often filled with vesicles that 

 are usually strongly drawn out in one direction. The remainder 

 consists of crystal grains. These, owing to their having higher 

 specific gravity than the glass fragment, were collected in con- 

 siderable quantity by mechanical concentration with water. 

 They consist of feldspar, hornblende, hypersthene, and a little 

 augite and magnetite. The large majority of the feldspars are 

 trielinic, as shown by the striation of basal cleavage flakes, and 

 the zonal structure of those lying on the brachypinacoid. In 

 composition they range from labradorite to oligoclase. Some 

 orthoclase is also present, as shown by the presence of basal 

 flakes with straight extinction, and unzoned clinopinocoidal 

 flakes extinguishing at an angle of about 5° from the base. 

 These crystals are often in the form of tablets elongated in the 

 direction of the clino-axis and twinned according to the Carlsbad 

 law by simple juxtaposition of the clinopinacoids. The hyper- 

 sthene is in well formed prisms; it shows strong pleochroism 

 in shades of green and brown, and straight extinction. The 

 hornblende is also well formed, is greenish brown and pleochroic, 

 and has an extinction angle of 17? 



A chemical analysis by the writer is presented here : 



