CALIFORNIA ACADEMY OF SCIENCES. 233 



Pitt, which is made up of huge blocks of rock, and nowhere 

 within thirty feet of the top can any rock be found in place. 



There has been but one phase in the volcanic activity of Mt. 

 Pitt, and the rock composing it is all basalt, the only variation 

 in it being that the rock composing the summit and upper por- 

 tion of the mountain, the more recently formed rock, is more 

 porphyritic in character and shows much vesicular inflation, 

 while the rock forming the lower part of the mountain and the 

 older flows is compact and fine grained. The former, though 

 varying somewhat in color, has usually a dark bluish gray paste, 

 thickly dotted with minute white crystals of feldspar, and dis- 

 seminated through it larger ciwstals of pyroxene, with occasional 

 grains of olivine. Although a solid firm rock, the vesicular infla- 

 tion is very pronounced, and when, as is frequently the case, 

 the vesicles are lined with a deep red, brown, yellowish or 

 greenish colored substance, presumably iron in different stages 

 of oxidation. The contrast of this deep color with the dark 

 bluish grey of the paste and the white of the feldspar crystals, 

 gives the rock a remarkably rich and varied appearance. When 

 much weathered, the red color spreads itself throughout the 

 mass of the rock, and sometimes the vesicular inflation is almost 

 pumaceous in character. The older rocks are of a lighter gray 

 color, frequently of a reddish tinge, compact, fine grained, and 

 apparently solid, though close examination shows them to be of 

 much the same general character as the others, and the vesicular 

 inflation is not wanting, but is so fine as to be barely noticeable 

 with the naked eye. Under the microscope these rocks show a 

 microcrystalline ground-mass, filled with clear porphyritic crys- 

 tals of plagioclase feldspar, pale green and brownish colored 

 pyroxene, fresh and unchauged, and some roundish grains of 

 olivine, sometimes quite fresh, at others edged with the thick 

 dark border so characteristic of this mineral. In the more vesi- 

 cular specimens there is more ground-mass, composed of lathe- 

 shaped crystals of feldspar and some glass, sprinkled with 

 minute grains of magnetite, while in the finer grained specimens 

 the rock is more evenly crystallized, and the ground-mass is less 

 prominent. The feldspar is all plagioclase, no orthoclase hav- 

 ing been observed. The jsyroxene mineral in these rocks con- 



