CHAPTER VI. 
GEOLOGY OE THE DES CHUTES BASIN. 
Compound nature of this basin.—Tts subdivisions similar in structure to those of pit and klamath rivers.—Trap plateaus.— 
Volcanic tufas and infusorial marls.—Local geology.—Plateau east of three sisters.—Canon of mpto-ly-as river — 
Mount jefferson.—Congealed lava stream.—Castle rocic—Columns of basaltic conglomerate capped with blocks of 
trap.—Tufaceous strata of mpto-ly-as river—Canon of psuc-ske que creek—Picturesque appearance of the colored 
TUFAS AND CONCRETE—TRUNKS OF CONIFEROUS TREES IMBEDDED IN TUFA.—COLUMNAR TRAP COVERING THE TUFAS.— -WAM CHUCK 
river.—Hot springs—Gelatinous silica.—Metamorphosed tufas.—Onyx.—Opal—agate.—Silicified wood—Wam chuck 
MOUNTAINS—MeTAMORPHIC SLATES—QUARTZ AND CHALCEDONY—NEE-NEE SPRING.—METAMORPHOSED MARLS—RlBBAND JASPER.— 
Plateau of tysch prairie.—Tysch mountains.—View of mount hood.—Canon of des chutes river.—-Mounds.—Hills of 
infusorial marl south of the Columbia. 
Although for convenience it may be desirable to group, under the name of the Des Chutes 
basin, the several distinct areas which are drained by the waters of the Des Chutes river, it 
should he stated that no such surface exists as would be indicated by its unqualified use. The 
several divisions which it must include have only in common their geological structure which, 
in all its essential particulars, they also share with the Klamath and Pit river basins. But 
while differing little in kind from that of the areas I have mentioned, the geology of the Des 
Chutes basin exhibits some striking features in the scale on which it is developed. Lying near 
the base of the chain of great volcanic cones which forms the axis of the Cascade range, immense 
quantities of erupted material have been thrown over it, which contrast strongly with the modest 
trap ridges, pumice plains, and fine chalk-like marls of the Klamath basin. 
The Des Chutes basin consists of a series of plateaus, having varying elevations from 4,000 
to 2,200 feet above the level of the sea, and being separated by subordinate ranges of volcanic 
mountains, of so low an elevation as scarcely to be noticed when overlooking the general surface 
from the Cascade mountains. These plateaus are usually covered by a floor of trap, which 
extends in a smooth sheet from fifty to a hundred and fifty feet in thickness, unbroken, except 
where crowning the slopes of the profound canons of the streams which traverse them. These 
layers of trap are frequently columnar, the columns being perpendicular. Below this stratum 
we find a series of volcanic marls, tufas, and conglomerates, locally intercalated with which are 
thin beds of trap. 
These tufaceous strata are, in many places, cut by the Des Chutes and its tributaries to the 
depth of more than a thousand feet without exposing the basis on which they rest. They are 
usually quite horizontal, from a few lines to twenty feet in thickness, and very accurately 
stratified. They exhibit great variety of color and texture, some being very fine and chalk¬ 
like, in all respects similar to those of Klamath and Pit river basins, while others are com¬ 
posed of fragments of pumice, volcanic sand, and a firmly cemented conglomerate of trap, 
pumice, scoria, and other erupted rocks. Some are pure white, others pink, orange, blue, 
brown, or green. The sections made by their exposures have a picturesque and peculiar 
appearance, of which some idea may he formed from the geological diagrams used in the lecture 
room. 
