GEOLOGY STRIPED SANDSTONE TYSCH MOUNTAINS. 51 



due to the regularity of their outlines, as well as to the entire absence of recent volcanic rock. 

 The slates which for the most part compose them are alumino-silicious, very hard and highly 

 metamorphosed, exhibiting the same general characters with those which form so prominent a 

 feature in the geology of the Cascade mountains west of its principal axis. They are divided 

 by deep, narrow ravines, and their slopes are long and steep, generally unbroken by any project 

 ing crag or perpendicular wall. The surface of these mountains is in many places strewed 

 with geodes and crystals of quartz, or masses of chalcedony, which have apparently filled 

 cavities in the rocks composing them. 



At Nee-nee Springs, several miles north of the Warn Chuck valley, stratified tufas here 

 somewhat disturbed and broken exhibit varieties of metamorphism not before noticed. What 

 was formerly one of the finer marls is here converted into a kind of fine-grained sandstone, 

 marked with ribbon-like lines of red and white. These seem to be the lines of deposition, and 

 indicate a periodical recurrence of the effects produced by two sets of causes. The red lines, 

 which are perfectly distinct sometimes not thicker than a sheet of paper ; more frequently 

 combining to form bands a quarter of an inch in width alternate with lines of white of about the 

 same width and of somewhat coarser texture. Small masses of scoria are disseminated through 

 the rock, and over these the lines of deposition are flexed, showing that the different bands 

 were formed by alternating layers of sediment the flexures of the lines of deposition over a 

 foreign body indicating, even in a hand specimen, which was the superior and which the 

 inferior surface. The general parallelism, and the continuity of the most delicate lines of 

 color, show that these sediments were deposited in tranquil water the bands of red indicating 

 the periods of most perfect quiet, when the finer materials, including a larger quantity of iron, 

 sank to the bottom. I have supposed it possible that the presence of iron in the red bands was 

 due to infusoria. If this material were carried through the same stages of metamorphosis 

 as much we have seen, it would form beautiful ribbon jasper. More perfect imitations of the 

 ribbon jasper of Germany and Egypt were, however, found at the Hot Springs, where a jaspery 

 rock was marked by bands of red and green. 



TYSCH PEAIRIE. 



North of the Warn Chuck mountains we came down on to Tysch prairie, which forms 

 a plateau precisely similar, in all its general features, to those we had previously traversed, 

 but lying at a lower level, having an altitude of but 2,200 feet above the sea. Mount Hood 

 rises from its western border, presenting an appearance remarkably imposing and beautiful, 

 well represented in plate No. IX, illustrating the general report of Lieutenant Abbot. From 

 the base of the Cascades it stretches eastward for thirty miles or more, forming a nearly level 

 plain, cut by the deep canons of Tysch creek and the Des Chutes. This plain is everywhere 

 underlaid by a stratum of trap, beneath which is a series of stratified tufas. 



TYSCH MOUNTAINS. 



The mountains which bound Tysch prairie on the north rise to an altitude of about 2,500 feet 

 above it. Their outlines are all rounded, and they are composed principally of compact trap, 

 not of recent date ; and of which all the rough and ragged surfaces have been worn away by 

 the action of the elements. 



Like Warn Chuck mountains, rising abruptly from the plateau which surrounds them, they 

 have a peculiar insular appearance. Like the Warn Chuck mountains, too, their slopes toward 



