GEOLOGY PIT EIVEE 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 exeeption or variation forms the mass of the range 

 where we crossed it. Through this barrier 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 Ihe exposed surface in many places retains the form 

 and appearance which it had when in a melted state. It is often bare ; at other times covered 

 with a thin soil, which has been formed by 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 by 

 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 



