gbegory.] ANDESITES. 169 
important particular from the type rocks of their class. Varietiesare 
found herej however, which are intermediate between andesites and 
trachytes, and some with dacite facies are also found. The exposures 
are numerous and eas} r of access, and the specimens arc no more altered 
in composition than if they were lavas of Tertiary age instead of 
Paleozoic. 
AUGITE - ANDE SITE . 
Macroscopic description. — The largest and best single exposure of 
andesite in this region is of the augite-andesite variety, and forms the 
main mass of Edmunds Hill. It does not occur as a solid, compact 
mass at any part of the hill, but is broken by cleavage and shearing 
planes into large blocks on top, and into plates and slated material at 
the ends of the hill. This slated and seemingly bedded appearance, 
which is so unusual in an igneous rock, is the most marked peculiarity 
of the structure of the hill. In a few places the rock is seen to con- 
tain embedded angular pebbles of glass and baked siliceous material, 
which stand out when it weathers; and in other places the rock pre- 
sents a banded surface of gray and brown, which gives the appear- 
ance of bedding, but which proves on examination to represent vary- 
ing stages of decomposition along potential cleavages. With these 
exceptions the exposed rock has a uniform appearance, gray Avhere 
weathered, black where fresh. 
Andesites are such well-known rocks that an extended macroscopic 
description is unnecessary. The hand specimen appeals as a black, 
basaltic-looking rock, generally dense, with a stringy effect and 
sprinkled over with glassy feldspars 2 millimeters and less in length. 
The weathered surface is a layer of spongy, gray-brown material, in 
which the pores are made by the decay of the larger feldspars. At the 
east end of the hill the rock is much lighter in color, and numerous 
white feldspars give it a more porphyritic appearance. 
Microscopic description. — Microscopic examination reveals the com- 
position and structure expected of a typical andesite. Magnetite, apa- 
tite, pyroxene, plagioclase, and orthoclase are the original minerals 
present. The plagioclase crystals range in size from laths 2 millimeters 
in length down to the very fine ones in the groundmass, but the larger 
ones are not abundant and do not give the rock a porphyritic aspect. 
The plagioclase forming the crystals outside the groundmass was 
determined by Michel LeVy's method to be labradorite; but the 
measurements indicated two labradorites, with the formula? Ah.An, 
and Ab 5 An 6 . The larger feldspars show strongly marked zonal band- 
ing, with occasionally as many as eight distinct zones, which decrease 
in basicity from the center outward, but with the original albite twin- 
ning running through the whole series. This albite twinning shows 
in nearly every feldspar lath with great distinctness, and twins on the 
