400 



J. ECCLES ON THE VOLCANIC ROCKS OF MONTANA, U.S.A. 



Fig. 3. — Map of part of the Tellovjstone Park. 

 (Scale about 20 miles to 1 inch.) 



is a linear distance of about twenty-five miles, but three days of hard 

 travelling. The rocks consist entirely of varieties of obsidian and 

 trachyte, the latter almost invariably being found in great mass 

 under the former. 



The obsidian, though frequent, is somewhat irregularly distri- 

 buted. It is both black and reddish brown, is occasionally columnar, 

 and is nearly always porphyritic. The most common form is a coarse, 

 rapidly weathering variety, containing many crystals of sanidine. 

 The disintegration of this rock produces some curious fine conglo- 

 meratic deposits in the old watercourses and river-beds (No. 8). 



The trachyte is constant and of very great thickness, and is evi- 

 dently elosely allied to the obsidian. A cliff-section in the Upper 

 Geyser basin suggests a transition from trachyte to obsidian. (Nos. 

 4 and 6 were obtained from this section.) 



