LITHOLOGY. 205 
and they will therefore be considered under the two heads of augite 
sienite, and hornblende sienite. 
Augite Sienite. In the neighborhood of the town of Jackson there are 
some gray-colored eruptive rocks, the true nature of which has never 
been understood, because it is only revealed by microscopic examinations 
of thin sections. In the region of the so-called tin mines, rocks are 
abundant which are coarse in texture, and apparently composed of ortho- 
clase and a dark fibrous hornblende. When a thin section of this rock 
is examined, it becomes plain that originally it was a compound of ortho- 
clase and augite, containing also much apatite and titanic iron; but the 
grains that were at first augite, are now in some cases partly, in others 
wholly, altered into a fibrous green mass of hornblende. This horn- 
blende, which results from the alteration of augite, was called uralite by 
Rose, because it was found abundantly in certain localities in the Urals. 
The augite is feebly red in color, and in all its cracks and cleavages the 
incipient alteration is seen, and from the outside the change is steadily 
progressing toward the centres. Fig. 5 on Pl. xi represents a grain of 
augite as it appears under the microscope. The well crystallized apatite 
prisms, the biotite, and titanic iron are also seen. The orthoclase is 
quite opaque, on account of its impure and decayed condition. In some 
cases chlorite and calcite are mixed with the hornblende. The iron 
oxide is often in those diffuse open forms which titanic iron usually. 
takes. Some of the grains are, however, very compact. Almost every 
one of the larger grains of the titanic or magnetic iron is surrounded by 
folize of biotite radially arranged in fan shapes, with the iron oxide as a 
nucleus. One of these grains is represented in Fig. 6 on Pl.xi. Itis pos- 
sible that the mica scales grouped themselves about the grain while the 
rock was plastic, or that they were formed there by a reaction between 
the silicates and the iron oxide. This is no isolated occurrence. In our 
gabbros the biotite and iron oxide are almost always associated, and I 
think that the mica was formed by a reaction in which the iron oxide 
took part, and therefore the minerals are thus associated, 
At a point one mile south-west of Mountain pond in Jackson, another 
warily of this rock occurs, which possesses some other features that are 
very interesting. This rock is finer in texture; the orthoclase is very 
much fresher and clearer; it contains some plagioclase, and also apatite, 
