F. L. Ransome — Lava Flows of Calif 



or ma. 



already indicated as the lower limit of the biotite-augite- 

 latite. 



Microscopic Petrography. — The rock making up the Table 

 Mountain flow is dark in color, and in large masses usually 

 exhibits columnar structure. Near Clover Meadow, however, 

 a portion of the flow has a pronounced spheroidal structure, 

 recalling forms which have been described in the altered Meso- 

 zoic basalts of Point Bonita.* In hand-specimens the rock is 

 dark grey, and shows numerous tabular phenocrysts of plagio- 

 clase up to 10 ram in length, and a few anhedra of augite, lying 

 in a compact aphanitic base. Most specimens show a few 

 small amygdules, and where the bottom of the flow is actually 

 exposed, the large plagioclase phenocrysts are often found 

 scattered through a highly frothy and glassy matrix. Under 

 the microscope it is seen that the phenocrysts, in the order of 

 their abundance, are large well-formed labradorites, often 

 spongy with glass inclusions, pale green augite, in more or less 

 rounded forms, and olivine, usually in small idiomorphic crys- 

 tals. These lie in a hyalopilitic groundmass consisting of 

 labradorile laths, grains of augite and a turbid globulitic glass. 

 Magnetite and apatite are present as accessory minerals. The 

 composition of the labradorite phenocrysts, as determined by 

 the method of Michel-Levy, varies from Ab,An, to Ab 3 An 4 , 

 and an examination of numbers of the larger laths in the 

 groundmass showed that they possessed approximately the same 

 composition as the accompanying phenocrysts. The rock of 

 this flow has been called an augite-latite. 



The biotite-augite-latite of the second flow differs con- 

 siderably from that just described. It varies from dark to 

 light gray in color, and contains abundant sparkling crystals of 

 biotite, and a few small labradorite phenocrysts, lying in a 

 somewhat porous groundmass. A noticeable feature in most 

 hand-specimens is the presence of irregular and frequently 

 elongated patches of dark glass, and of small pebble-like inclu- 

 sions, giving the rock at times an almost tuffaceous aspect. 

 Under the microscope, typical facies present a pronounced 

 eutaxitic structure. Phenocrysts of labradorite, biotite, and 

 augite lie in a groundmass in which streaks, patches, and flam- 

 boyant tongues, of comparatively clear glass, alternate with 

 more turbid areas in which incipient crystallization had already 

 made some progress at the time when the lava solidified. The 

 small inclusions appear in the majority of cases to be more 

 fully crystalline portions of the same magma, which, having 

 solidified, were broken up and involved in the flow bv the for- 

 ward movement of the viscous mass. Other larger inclusions 



* University of California, Bull, of Geol., vol. i, pp. 75-80. 



