660 ERUPTIVE ROCKS FROM MONTANA— MEIIRILL. vol.xvii. 



nearly black, and in structure from coarsely vesicnlar to compact, and, 

 as a rule, showing olivines developed in such sizes as to be recogniza- 

 ble to the unaided eye. 



As a rule the samples collected present no points of exceptional 

 interest, though from an outcrop on the divide between the two south 

 branches of Moore Creek a quartzose variety was found which needs 

 mention. Macroscopically the rock is dense, compact, of a dark-gray 

 color, and studded with numerous rounded or oval spots, 2 to 3 mm. in 

 diameter, showing a whitish center surrounded by two narrow zones, 

 the inner greenish in color, and the outer, an irregular and imperfect 

 one, whitish. In the thin sections these spots show a rounded nucleus 

 of quartz surrounded by a zone of pale-green augite, and these in turn 

 surrounded by a zone of nonstriated feldspars ( ?). The nature of this 

 last constituent could not be made out beyond doubt in the sections at 

 hand. The mineral is biaxial and gives inclined extinctions, the gen- 

 eral behavior being that of a potash feldspar. With the exception of 

 this imperfect outer zone the occurrences are apparently in every way 

 similar to those described by Diller* and Iddings, tand are to be 

 accounted for in a similar manner. 



Hornblende andesite. — Old Tollhouse on road leading from Postle- 

 waite Creek toward Virginia City (:N"o. 72807, U.S.N.M.). This is a 

 gray andesite of ordinary type, showing to the unaided eye coal black 

 hornblendes, hexagonal folia oif black mica from one to two mm. in 

 diameter, and abundant small i^lagioclase phenocrysts. The microscope 

 brings to light no points of unusual interest. The rock is finely exposed 

 in the hillside at the tollhouse, and is found to underly the basalt 

 forming the plateau to the west. The same rock occurs again at Vir- 

 ginia City, where it has been used in the construction of several build- 

 ings. On the east side of Alder Gulch, also underlying the basalt, a 

 similar rock occurs, but in which the hornblende seems to have been 

 wholly replaced by the black mica. 



Liparites. — Cherry Creek on west side of Madison Valley. The 

 only eruptives here (N^o.72945, U.S.N.M.) are liparites and diabases, the 

 first occurring only in remnants of thin sheets on the slopes north 

 of the creek, and in isolated patches for several miles to the south- 

 ward. The prevailing type is a light reddish or gray and but slightly 

 porphyritic rhyolite, sometimes coarsely spherulitic. The material is 

 of such slight density as to have been transported by spasmodic 

 streams clear to the opposite side of the valley in masses of even 

 10 feet in diameter. Wind blown sand has in many cases hollowed 

 these out into a mere shell. Older eruptives in the form of dikes of 

 diabase occur well down in the edge of the valley, outcroiDS running 

 parallel with the prevailing schists. These in the hand specimens are 

 holocrystalline granular rocks, dark gray in color, and in which an 



*Am. Jour. Sci., xxxiii, Jan., 1887, p. 45. 

 t Am. Jour. Sci., xxxvi, Sept., 1888, p. 208. 



