82 



University of California. 



[Vol. i. 



shows smali amygdules of calcite and chlorite, the latter about 0.75 

 mm. in diameter. Under the microscope it is seen to consist of a 

 groundmass of lath-shaped and acicular plagioclase and glass, with 

 some sharply bounded phenocrysts of plagioclase, about 2 mm. in 

 length. The plagioclases of the groundmass seldom show rectan- 

 gular terminations, but fray out in sharp, jagged points. Glass occu- 

 pies a large proportion of the slide. Small black specks of mag- 

 netite are sparingly disseminated through the rock, and there is 

 considerable secondary chlorite. 



About thirty feet farther north, a specimen was taken almost 

 identical in macroscopic appearance with the one just described, and 

 certainly from the same general rock-mass. It presents under the 

 microscope, however, a somewhat different appearance, though 

 having the same general texture of groundmass. The most im- 

 portant difference is the presence of augite in small irregular grains 

 and plates. It is not perceptibly pleochroic, extinguishes at a maxi- 

 mum angle of about forty-five degrees from the prismatic cleavages, 

 and is younger than the plagioclases. There is also a relatively 

 greater abundance of small, opaque, black granules, and as some of 

 these are surrounded by gray clouds, they afford the first hint of the 

 presence of titanium. Calcite and chlorite are abundant, filling 

 cracks, amygdules, and interstitial spaces. 



The next rock to be described in detail is that forming the large 

 spheroids shown in Plate 7, Fig. 2, at the turn of the path already 

 referred to. It is the most conspicuously porphyritic rock in the 

 area, and weathers to a rough surface, on which the feldspars, vary- 

 ing in length from an eighth to three-quarters of-an inch, stand out 

 in relief. Upon fresh fracture the rock is dark greenish gray in 

 color, showing the feldspathic phenocrysts imbedded in a compact 

 groundmass dotted with minute dark-colored amygdules. The 

 large feldspars are seen with the naked eye to be full of dark inclu- 

 sions, sometimes exhibiting a zonal arrangement. The large sphe- 

 roid in the illustration was broken completely into fragments, but the 

 rock showed the same structure throughout, with the exception of 

 being less amygdaloidal near the center; it thus differed markedly 

 from the "lava-balls" described by Dana* in the Hawaiian Islands. 



* Characteristics of Volcanoes, pp. 9-1 1. 



