158 The American Geologist. September, 1897 
increase in acidity from the center to the periphery, in others 
a succession of basic and more acid layers. This latter is 
the more common of the two. 
In most of the slides the hypersthene preponderates over the 
aiigite. Its microscopic characters are as usual. As is the 
rule in andesites the pleochroism is rather faint for this 
mineral, with very slight difference between a and b. Par- 
allel growths with augite are of frequent occurrence, the brach- 
ypinacoid of the hypersthene in contact with the orthopin- 
acoid of the augite. Both magnetite and glass occur as in- 
clusions, but are infrequent. 
The augite phenocrj^sts are of a pale yellowish-green color, 
intermediate between the pale greenish and pale greenish-yel- 
low of the hypersthene. A very slight pleochroism is appar- 
ent, the yellowish tinge being somewhat more pronounced par- 
allel to a. The extinction angle is 48*^. Inclusions are the 
same as in hypersthene. 
The groundmass. The eleven specimens show considerable 
variation in the texture and structure of the groundmass. 
This is also probably true of the chemical composition, the 
material furnished being insufficient for analytical purposes. 
In five specimens there is little or no glass and one of them is 
quite coarsely crystalline, made up of short, thick bytownite 
crystals and prisms of augite and hypersthene, the whole 
thickly set with magnetite dust. The remainder are similar 
except for the smaller size of the crystals. 
Of the remaining six specimens two show ordinary hyalo- 
pilitic structure, while in the other four (the pumices and 
pitchstone) the groundmass is wholly glassy. 
The specimens also range from a rock with basalt-like 
groundmass, with much pyroxene and magnetite, and feld- 
spars in the labradorite-bj^townite series, to a rock with tra- 
chytic groundmass, the feldspar laths composing it belonging 
to the oligoclase-andesine series and the pyroxenes only occur- 
ring as phenocrysts. 
Enclosures. The slide from specimen 8 shows a small en- 
closure of fresh diabase indicating its presence somewhere in 
the path of the lava. 
Two other slides show enclosures of small size made up of 
