46 



GEOLOGY — VOLCANIC TUFAS AND INFUSORIAL MARLS. 



on either side from the immediate vicinity of the stream which flowed at the bottom. It had 

 every appearance of having been excavated from the solid rock which forms its sides. The 

 stream which flows through it is thirty yards in width, three or four feet deep, and very rapid. 

 It is formed by the drainage of Mount Jefferson and the Three Sisters. 



CONGLOMERATE COLUMN, MPTO-LY-AS RIVER. 



At the point where we left the canon of Mpto-ly-as river a marked change occurred in the 

 character of the material composing its walls. The precipices, composed of trap and volcanic 

 conglomerate, which with a height of nearly 2,000 feet had enclosed it for twenty miles, were here 

 succeeded by strata of tufas, which formed walls of perhaps 1,200 feet in height, capped by a 

 thick layer of columnar trap. These tufas were nearly horizontally stratified ; exhibiting all the 

 varieties which I have described ; the different strata varying in thickness from a few inches to 

 twenty feet. 



Some of the finer varieties are highly infusorial. The forms which they contain have since 

 been examined by Professor Bailey, who pronounces them indicative of a fresh water origin. 

 We here ascended to the north wall of the canon, travelling over the plateau to the banks of 

 Psuc-see-que creek, another tributary of the Des Chutes, flowing down from Mount Jefferson. 

 Here we found a similar series of tufas apparently quite undisturbed, their strata horizontal 

 and continuous. 



Mingled with these tufas, at this point, are many strata of conglomerate, of which the base 

 resembles closely Roman cement; the inclosed pebbles, usually of small size, and of all varieties 

 of volcanic rock. These beds of concrete being harder than the associated strata, have, in the 

 erosion of the canon, formed successive steps, frequently thirty or forty feet in width. The 

 detached fragments of these layers of concrete cover and protect pinnacles of the softer stratum 



