GEOLOGY-PIT RIVER VALLEY. 
35 
passage through its eastern wall. The sediments deposited by its waters, which form the 
surface over which we passed, though for the most part undisturbed, and nowhere exhibiting 
the rearrangement which marks most of the sediments of the Sacramento valley, closely 
resembled the strata of fine infusorial marls which are found in various parts of that valley, 
and which seems at one time to have stretched over a large portion of its surface. 
RANGE FORMING UPPER CANON OF PIT RIVER. 
The geological structure of this range presents a striking similarity to that which connects 
Lassen’s butte and Mount Shasta. The dark vesicular trap, which forms the lower canon of 
Pit river, here reappears, and almost without exception or variation forms the mass of the range 
where we crossed it. Through this harrier Pit river has forced its way in a narrow and some¬ 
what tortuous canon, of which the perpendicular walls present sections frequently several 
hundred feet in height. The surface rock on the north side is everywhere the dark vesicular 
trap to which I have referred, and of which the exposed surface in many places retains the form 
and appearance which it had when in a melted state. It is often hare ; at other times covered 
with a thin soil, which has been formed hy its decomposition. It presents very few level 
surfaces ; is covered with a thin growth of coarse grasses, with here and there a dwarfed tree 
of the western cedar. On the south side of the canon the rock is generally similar in character, 
but near the middle of the range I noticed a mass of red and apparently recent scoria. 
SECOND BASIN OF PIT RIVER. 
Descending the eastern side of the range of which I have been speaking, we came down on 
to a second plain, similar in all respects to that which lies westward of it. It has nearly the 
same breadth, about twenty miles ; its longest diameter being parallel to the mountain range 
which borders it, its limits north and south not being visible from any point of our route. 
Like the lower basin, it is very nearly level, and lies at an average altitude of a little over 4,000 
feet, being 800 feet higher than the one we previously crossed. The drainage of this plain is, 
apparently, less perfect than that of the lower one; it is more moist; covered with a deeper 
soil, sustaining a more vigorous growth of green grasses; and, from the number of fluviatile 
shells strewed over its surface, is evidently at some seasons overflowed. We had little oppor¬ 
tunity of examining the structure of this plain, but it is apparently generally underlaid bjr 
infusorial marls similar to those already described. In the vicinity of the hills which border it 
on the east these marls appear in various localities, considerably elevated above its level, and 
have, apparently, been subject to some disturbance since their deposition. The most common 
form which they here present is precisely like that which occurs in the lower canon of Pit river, 
being as fine and white as chalk, and like that abounding in the remains of fresh water infu¬ 
soria. Associated with this are strata of soft green sandstone, which occurs in thin beds inter- 
stratified with the last. 
The hills of which I have spoken, as forming the eastern limit of this plain, scarcely deserve 
the name of mountains, and in the imperfect examination I was able to give them I was unable 
to detect the course of the lines of upheaval by which they had been formed. They exhibit 
considerable variety in the rocks which compose them, which are, however, all erupted or highly 
metamorphic. A dark compact basalt, greenstone, and porphyry are all present, and among the 
boulders found in the bed of Pit river, apparently derived from these hills at a higher point 
in its course, I found jasper, agates, quartz, granite, porphyry, and obsidian. The hills which 
