CHAPTERVI. 



GEOLOGY OF THE DES CHUTES BASIN. 



Compound nature of this basin. — Its subdivisions similar in strccture 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 rock. — Columns of basaltic conglomerate capped with blocks of 

 trap. — Tufaceous strata of mpto-ly-as river. — Canon of psuc-see-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 tbe name of tbe Des Chutes 

 basin, the several distinct areas which are drained by the waters of the Des Chutes river, it 

 should be 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 sbeet from fifty to a hundred and fifty feet in thickness, unbroken, except 

 where crowning the slopes of the profound canons of the streams wbich 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 be formed from the geological diagrams used in the lecture 

 room. 



