fi54 ERUPTIVE BOCKS FROM MONTANA— MERRILL. voL.xvn. 



it was supposed to be a basalt. The thin section, however, shows only 

 a very dense aggregate of small feldspars and other colorless microlites, 

 often with strongly marked fluidal structure and innumerable small 

 opaque specks, assumed to be iron oxides. None of the magnesian sili- 

 cates are sufficiently develojjed to be recognizable, even with the high- 

 est powers. The white jiorphyritic constituents mentioned above as 

 occurring rarely, prove to be secondary segregations of quartz. 



The microstructure of the rock is andesitic rather than basaltic; 

 nevertheless, as I have never before seen any but the glassy andesites 

 so dark in color and so compact, a test was resorted to which showed 

 64.42 per cent, of silica; specific gravity, 2.555. 



Hornblende picrite. — North Meadow Creek. This rock (No. 73174, 

 U.S.N.M.) in the hand specimen is of an iron-gray color and com- 

 posed of very irregularly outlined crystal plates and fibers, from one 

 to two centimeters broad, which to the unaided eye merge into one 

 another without sharp lines of demarkation. The larger plates inclose 

 rounded blebs of a deep green mineral, whereby is produced an indis- 

 tinct and irregular luster-mottling. The rock weathers brownish on 

 the immediate surface, but is apparently fresh and unaltered and so 

 intensely tough and hard that a two-pound hammer quite failed to 

 detach chips of any size, and recourse was had to the sledge. 



The outcroiis are few and rather inconspicuous, projecting iu thin 

 wedges but a few feet above the scanty soil, and ringing like metal 

 when struck with the hammer. Although no contacts were visible, the 

 strike is directly across that of the gneiss in the vicinity, and in the 

 field no doubt was felt concerning its eruptive nature. 



In the thin section, under the microscope, the rock shows a holocrys- 

 talline aggregate of light greenish to colorless hornbleudes, abundant, 

 beautifully fresh, and colorless olivines, irregular grains of pleonast, the 

 usual sprinkling of iron ores, and occasional very imperfectly outlined 

 areas of a faint brown, dichroic mineral, showing in an indistinct basal 

 section a very irregular and interrupted nearly rectangular prismatic 

 cleavage. Satisftictory determination of this mineral was impossible. 

 It is evidently hypersthene. The hornblendes occur in broad ophitic 

 plates of a green color, inclosing the rounded clear and fresh olivines, 

 and also in colorless frayed-out asbestus-like forms. Were it not that 

 the rock is so fresh and unaltered I would be disposed to regard such 

 as secondary forms, x)erhaps after a rhombic pyroxene, but it seems 

 scarcely probable that such an alteration could have taken place, leav- 

 ing the olivines, which they inclose, so perfectly fresh and unchanged. 



The rock belongs to the group of hornblende picrites, as defined by 

 Bonney, and though a trifle more acid, seems to correspond fairly well 

 with those described by him from Anglesea and figured by Teall on 

 pis. v and VI, figs. 2 and 1, respectively, of his British Petrography. 

 The rock is sufficiently rich in olivine to gelatinize readily in hydro- 



