8o 



University of California. 



(Vol. i. 



forms a small rounded projection jutting into the waves from under 

 a mass of the ordinary diabase, the two rocks being separated by a 

 sharp line of contact, giving the impression that the latter rock, 

 while in a molten condition, moulded itself to the irregularities of 

 the former. The same peculiar forms are even more pronounced 

 here than at F. » 



Microscopic Petrography.— Somewhat contrary to expectations, 

 the spheroidal basalt did not exhibit any features of exceptional 

 interest under the microscope, the sections showing quite uniformly 

 a finely crystalline amygdaloidal rock, composed of lath-shaped 

 feldspars and a considerable proportion of glass. Porphyritic plag- 

 ioclases are sometimes present, but are not a constant feature. 

 Secondary alteration is generally well advanced, and the rock is per- 

 meated with calcite, chlorite, and sometimes quartz. There can 

 generally be observed a well-defined flow structure, exhibited in the 

 arrangement of the lath-shaped feldspars with their longer axes 

 rudely parallel, in their tangential position around the vesicles, in 

 their frequent curved form, and in the faulting of the larger porphy- 

 ritic crystals. As a general rule the lath-shaped feldspars appear as 

 single individuals, without polysynthetic twinning ; but such extinc- 

 tion angles as were measured, the polysynthetic twinning of the 

 fresher porphyritic feldspars, and the low silica percentage of the 

 rock — less than 50 per cent — indicate that they belong near the 

 basic end of the plagioclase series. 



At the lighthouse, the rock exhibiting the structure shown in 

 Plate 7, Fig. 1, is light grayish green, on fresh fracture, with a very 

 compact aphanitic texture. It has been much silicified, and is trav- 

 ersed by a multitude of fine quartz veinlets, together with larger 

 veins of calcite. This veining, combined with the compact porcelain, 

 like texture, renders the rock very brittle, and causes it to fly into 

 small fragments under the hammer. A thin section cut from rock 

 forming the exterior portion of one of the bales, showed it to be 

 composed of small lath-shaped plagioclases in a glassy base, the 

 whole being somewhat obscured by secondary products. Small 

 round amygdules are fairly abundant, generally showing a quartz 

 border with a central filling of chlorite. A section from the interior 

 of one of the bales showed a rock similar to the foregoing, but dif- 



