38 



GEOLOGICAL SURVEY OF THE TERRITORIES. 



with tlie tufas. Id Fig. 3 we see a curious instance of a sort of uucon- 

 formability, showing, i)robably, the periods of deposition. The under- 



Fig. 3. 



VOLCANIC TUFFS AND BRECCIA, YELLOWSTONE VALLEY. 



lying portion had .been worn into a conical shajye by the waters of the 

 lake prior to the deposition of the slanting mass. In the back-ground 

 there is a lofty range of volcanic breccia or conglomerates, rising 3,000 

 feet above the valley. 



We will now take our leave of this beautiful valley and proceed up 

 the river tbrough the Second Caiion. We may hastily notice the char- 

 acter of the rocks on either side. The divide between the Yellowstone 

 and the Gallatin is quite sharp and narrow, the numerous little streams 

 cutting deep channels down from the crest to the river. The divide 

 itself is formed entirely of the volcanic conglomerate, weathered into 

 the most siuguhir architectural forms. West of the canon, at the head 

 of Canon and Eock Creeks, this conglomerate is at least 1,000 feet 

 thick, horizontally stratified, and worn into hundreds of pinnacles or 

 Gothic columns. (See Fig. 4.) These conglomerates extend down the 

 ridges between the little streams for several miles, while the gorges are 

 cleft deep down to the metamorphic rocks. The caiion itself, on either side, 

 is composed entirely of gneissic strata. Silicified wood is found in these 

 conglomerates in great quantities, and sometimes huge logs and stumps 

 are exposed in a vertical position in the walls. Near the head of Caiion 

 Creek, I saw the stump of a tree which must have been at least thirteen 

 or fourteen feet in diameter originally. It was firmly inclosed in the 

 breccia. The question arises in the mind. Whence originated this vast 

 deposit of breccia or conglomerate, and what were the physical condi- 

 tions under which the niaterials were deposited t As to their origin, we 

 must conclude that they were thrown out by volcanoes into the sur- 

 rounding waters much as similar materials are ejected from modern 

 volcanoes at the present time. We find, however, that these breccias 

 are of immense thickness, sometimes 4,000 to 5,000 feet, as at the 



