LITHOLOGY. 20$ 



and they will therefore be considered under the two heads of augitc 

 sienite, and hornblende sienite. 



Aiigite 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 PI. 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 

 folias of biotite radially arranged in fan shapes, with the iron oxide as a 

 nucleus. One of these grains is represented in Fig. 6 on PI. xi. It is 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 

 variety 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, 



