GEOLOGICAL SURVEY OF THE TERRITORIES. 



141 



Springs at Mud Volcanoes. 

 General elevation above sea-level, 7,775 feet ; boiling-point 198°. 5 F. 



CHAPTER IV. 



GEYSER-BASINS OF EIEE-HOLE EIYEE. 



We left Mud Volcanoes on the 13tli of August and started for tbe 

 lower geyser-basin of Fire-Hole River. Our course at first led us 

 up an open valley that once formed part of the ancient bed of Yellow- 

 stone Lake. At the bead of the valley we struck our old trail of last 

 year, which we followed until we reached the east fork of the Fire-Hole 

 River. The divide between the Y^'ellowstone River and the Fire-Hole 

 River at this point is 8,164 feet above sea-level. The summit seems to 

 be made up mostly of obsidian, which is all porphyritic. The timber is 

 so thick that it is difficult to trace the connection, but, as we descend, we 

 come across trachytes that seem to underlie the obsidian. It is very 

 compact and porphyritic, containing crystals of sanidine. The general 

 color is a light blue, approaching violet. It is through these trachytes 

 that the iieadwaters of the Madison cut their channels. Near the sum- 

 mit, on the Madison side of the divide, there is an old hot-spring basin in 

 which the springs are now all extinct, although there are a great many 

 steam-vents from which steam still escapes; these vents are lined with sul- 

 phur. Besides sulphur the deposits consist mainly of silica and iron, 

 ffhe descent from the divide is very steep and rocky, and through thick 

 timber, a great deal of which is dead and fallen. The valley of the east 

 fork of the Madison or Fire-Hole at the point we reached it is very 



