34 CALIFORNIA ACADEMY OF SCIENCES. [PROC. 3 D SER. 



The feldspar phenocrysts occur in idiomorphic sections 

 and are usually lath-shaped, presenting a tabular develop- 

 ment parallel to the brachypinacoid. There is a slight tend- 

 ency to the formation of ruin-like terminals. The crystal 

 boundaries are in many cases clear and sharp, though most 

 of them show a varying amount of resorption both on the 

 sides and terminals of the sections, oftener the latter. Some 

 of the feldspars clearly show a second period of growth 

 after having been in part resorbed into the magma. Zoning 

 is rather common. Twinning is in accordance with both 

 albite and Carlsbad laws. Occasional cracks penetrate the 

 sections and some of these are brought out more clearly by 

 a staining of limonite. In sections cut approximately per- 

 pendicular to the albite lamellation the extinction angles 

 show the species to be labradorite. As a rule the sections 

 of the feldspar are very fresh and clear, being free from 

 decomposition products. Inclusions are not uncommon, 

 consisting chiefly of brownish glass with a few small and 

 irregular pyroxenes and occasional small grains of magnet- 

 ite. The glass is usually in irregular patches and is either 

 centrally or zonally arranged. In some cases it occurs in 

 roughly rectangular forms zonally arranged, and with their 

 longer axis parallel to the longer axis of the crystal. The 

 length of the sections of labradorite varies from perhaps 

 .06 mm. to about 1.5 mm. The sections are usually quite 

 numerous, though their number is very variable, and some 

 of the specimens show few phenocrysts of any sort. 



The phenocrysts of the various minerals generally occur 

 as scattered crystals, but they occasionally form small ag- 

 gregates. In this case, the boundaries of the feldspars yield 

 to those of the ferromagnesian minerals with which they 

 are in contact. This, with the occasional complete inclu- 

 sion of these minerals already mentioned, shows the later 

 development of the feldspars. 



The ground-mass consists largely of feldspar microlites 

 with a varying amount of interstitial glass. Pyroxene oc- 

 curs occasionally in flakes in the ground-mass, when but lit- 

 tle developed porphyritically. The feldspars have a low 



