II GEOLOGY VACAVILLE TO CHICO CREEK. 



crystalline and metamorphic, and contains no fossils. From its relations to the limestone dis- 

 covered by Dr. Trask, near the base of Mount Shasta, and which is of carboniferous age, one 

 may suspect it to belong to that era. 



The great mass of the Sierra Nevada is composed of plutonic or volcanic rock, granite, 

 gneiss, mica schists, and porphyrys, trap, trachyte, &c, with auriferous talcose slates, and 

 veins of quartz. These strata, having been extensively broken up and eroded by aqueous or 

 glacial action, have, in the re-arrangement of their constituent materials, given rise to the 

 placer deposites which skirt the base of the range. The deposition of this comminuted material 

 has apparently been effected by aqueous agency, and controlled in a degree by the law of 

 gravitation, as the gold, the heaviest of the component materials, is found at or very near the 

 bottom. The surface of the plain which lies between these ranges of mountains is underlaid 

 by beds of transported material — gravel, clay, and tufaceous conglomerate — several hundred 

 feet in thickness, which were once deposited as sediments on the bottom of the trough, but 

 have been extensively re-arranged by the present water-courses, and in many places subjected 

 to considerable disturbance from volcanic action. 



LOCAL GEOLOGY. 



VACAVILLE TO CHICO CREEK. 



After leaving the foot hills of the coast mountains, we traversed the valley diagonally to the 

 vicinity of its eastern margin. Making this transit, we were constantly upon the alluvial 

 deposits which have been referred to, and nowhere found any rock in place on the immediate 

 line of our march. Between the base of the hills, near Vacaville and Putos creek, the surface 

 passed over formed low hills and table land composed of gravel, entirely destitute of trees, but 

 covered with a thin coating of the grasses and other plants which have been mentioned as 

 characteristic of the gravel surfaces of the valley. The soil has apparently but little fertility, 

 and is nowhere grazed or cultivated. The pebbles which compose the gravel beds are generally 

 of small size, well rounded, and consist of jasper, quartz, porphyry, trap, &c. As we 

 approached Putos creek the soil became fine, loamy, and fertile, and on the banks of the 

 stream supported a narrow belt of magnificent oaks. 



The banks of the stream are distinctly terraced, the upper bench being some 25 feet above 

 the lower, which is about the same distance above the bed of the creek. The material of 

 which these lower terraces is composed is principally a fine alluvial earth, mingled with which 

 are a lew pebbles. The upper terrace consists in greater degree of pebbles, some of which are 

 of considerable size, much rounded, and, like those found in the bed of the stream, composed of 

 trap, jasper, and quartz. 



Cache creek. — The interval lying between Putos and Cache creeks is similar in its features to 

 that south of the former. The immediate vicinity of Cache creek, however, is a region of great 

 fertility ; the soil is dark and deep, and the belt of timber which borders the stream is wider, 

 and the trees even finer than those of Putos creek. These differences are doubtless mainly due 

 to a more abundant supply of water afforded by Cache creek. 



Its banks are alluvial, the bed gravel, the current rapid, and the water clear and good. 

 The terraces of Cache creek are not as perceptible as those of the Putos, the upper bench being 

 further removed from its immediate banks. The region lying between the crossing of Cache 

 creek and the Sacramento, at Knight's Landing, is very level and nearly all under cultivation. 

 Unlike the country previously traversed, we found this not covered with wild oats or dried 



