OF THE TITKEOTJS ROCES OF MONTANA, U.S.A. 



399 



of the ground-mass. A little bright green matter is also present, 

 which appears to occupy irregular vesicles, and which is either green 

 earth or some closely allied mineral of secondary origin. The angles 

 of extinction of the felspars seem to indicate that the triclinic ones 

 are labradorite. 



The rock from Tower Palls closely resembles that just described, 

 except that it contains apparently no augite and more hornblende. 

 The crystals of the latter mineral are also surrounded by opaque 

 borders, which are often broad, and consist of an internal zone of 

 magnetite and an external one of haematite, which appears sharply 

 defined at its contact with the magnetite. The rock is hornblende 

 andesite. It is of paler colour than that from Mount "Washburn, 

 the hand-specimen being grey, with minute reddish brown, black, 

 and white specks *. 



APPENDIX. 



On the Mode of Occurrence of some of the Volcanic Kocks of 

 Montana, U.S.A. By James Eccles,- Esq., P.G.S. 



The volcanic district of the Yellowstone National Park, from which 

 I obtained the various rock-specimens referred to in the foregoing 

 paper, has been already described in detail by Dr. Hayden, Dr. A. 

 C. Peale, and Prof. Bradley, in the reports of the U.S. Geological 

 Survey of the Territories for the years 1871 and 1872. 



In the autumn of 1878 I had the good fortune to accompany Dr. 

 Hayden and other members of his survey in some parts of the 

 district referred to ; and although I have no intention of giving a 

 detailed description of the volcanic phenomena which were observed, 

 a short notice of the localities whence the specimens were obtained, 

 and of the general mode of occurrence of the rocks, may be of some 

 interest as a supplement to Mr. Eutleys description of their micro- 

 scopic characters. 



The Yellowstone National Park, within which are the head-waters 

 of two of the great forks of the Missouri (the Yellowstone river and 

 the Madison) and of the Snake river (one of the branches of the 

 Columbia, comprises an area of about 3600 square miles, nearly the 

 whole of which is covered up by volcanic rocks of great thickness. 

 There are very few exposures of underlying sedimentary or other 

 formations : and these are almost entirely limited to the extreme 

 northern edge of the area. Some fifteen miles south of the southern 

 boundary the extension of these volcanic rocks is seen to rest upon 

 the northern spurs of the Teton range of mountains, at which point 

 the underlying formation has been ascertained by Prof. Bradley to 

 be of Carboniferous age. On approaching the Park from the south 

 along the upper valley of the Snake, these volcanic rocks appear to 

 form an irregular plateau densely covered with forest. Prom the 

 point at which we first struck this plateau, as far as the Upper 

 Geyser basin on the Pirehole river (the chief branch of the Madison), 



* For the discussion upon this paper see p. 412. 



