46 



University of California. 



[Vol. 2. 



arrangement of the laths is frequently fluida] about the amygdules. 

 The laths sink to exceedingly fine microlites in some facies, so that 

 their outlines are hardly distinguishable. One example was noted 

 in which the mass of the slide is made up of spherulitic feldspars, 

 the field being completely dotted with poorly defined dark crosses 

 which remained stationary on the rotation of the stage. In this 

 matrix crystals of idiomorphic feldspar were disseminated. The 

 extinction of the feldspar laths is generally very small, but occa- 

 sionally reaches 23 . It is probable that much of the glass of 

 these rocks has become devitrified, as the amount detected is com- 

 paratively small. Quartz is very abundant in nearly all the slides. 

 It not only constitutes the chief filling of the amygdaloidal cavities, 

 but is also thickly scattered in irregular grains, apparently filling 

 interstitial spaces between the feldspar microlites. Its secondary 

 character is probable, though there are cases in which it closely 

 simulates primary quartz. Infiltrations of epidote and calcite fol- 

 lowed the quartz. Calcite is abundant through the matrix of many 

 portions of the basalt. Magnetite is usually abundant in dusty 

 grains. 



The dark massive eruptives which have broken through the 

 amygdaloidal lavas appear under the microscope more nearly 

 related to diabases than to basalts. They consist of a very fine- 

 grained mixture of lath feldspars and augite, which apparently crys- 

 tallized almost simultaneously. The feldspars have the same 

 extinction angles as in the amygdaloidal rocks. There is a tend- 

 ency to the development of porphyritic crystals of feldspar and 

 augite which are more nearly idiomorphic. Secondary products 

 such as chlorite, calcite, and quartz are abundant, while no glass 

 could be detected. An analysis of one of these masses, intrusive 

 in the spheroidal basalt, unexpectedly showed a high percentage of 

 silica and little lime, allying it to the andesites. The texture of the 

 dikes cutting the gabbro is coarse in proportion to their size, and 

 they are often much finer on the edges than in the center. A slide 

 made from a dike two feet wide showed an exceedingly fine-grained 

 base, consisting of feldspar microlites and minute grains supposedly 

 augite. In this appear scattered feldspars of larger dimensions. 

 Much of the groundmass is isotropic 



