VICINITY OF OROVILLE AND CHEROKEE FLAT. 



481 





eiice of rim-rock, may be described as follows : Beginning at a point in Saw Mill Ravine, near 

 the Spring Valley House, the rim has a general northeasterly direction for about a mile ; it then 

 sweeps around and returns parallel to its former direction for nearly half that distance, enclosing a 

 bay-like mass of gravel 1,500 feet in width, and finally follows a southerly direction, passing 

 a quarter of a mile to the east of the Sugar Loaf. The grade of the bed-rock where exposed to 

 the north of the Sugar Loaf, is very steep from northeast to southwest. At the road-crossing, near 

 the high northeastern extremity of the workings, I made the altitude of the bed-rock to be 1,300 

 feet, while that of the bed-rock near the blacksmith-shop in the lower mines is only 1,087 feet, 

 the distance between the two points of observation being less than a mile. To the south and west 

 of the blacksmith-shop the position of the bed-rock is not known, but it is certainly fully as low 

 as that whose altitude has just been given. The bed-rock northwest of Cherokee Flat is principally 

 slate, but at the mine several distinct varieties of rock, not all of them slaty in structure, are rep- 

 resented. The best place to study the relations of these rocks would be in one of the tunnels, at a 

 time when there was no work doing in the mine. The rocks met with in driving the Eureka 

 tunnel were, as I was told by the superintendent, as follows : At the mouth of the tunnel in 

 Saw Mill Ravine the rock was first a soft slate, which gradually grew harder for a distance of seven 

 hundred feet; then came four or five hundred feet of an extremely tough rock, something like 

 pudding-stone ; after this for five or six hundred feet the rock was soft, easily picked, and greenish 

 in color ; the remainder of the two thousand feet of the tunnel was again in a very hard rock. 

 Welch's tunnel, which is higher and shorter, is mostly in slate. 



At the bottom of the gravel next the bed-rock there is a stratum, from ten to thirty feet in 

 thickness, blue in color, resembling the " blue gravel " of the mines of Yuba and Nevada counties, 

 more or less cemented together, especially near the bottom, and containing in its lower portions 

 large boulders, which are not much rounded. Some of these boulders, indeed, are decidedly angu- 

 lar, and some of them are quite flat. The color of this stratum as a whole is due largely to the 

 prevailing bluish color of the boulders ; for between them, and frequently cemented to them, the 

 gravel is composed of fine, white, rolled quartz. The upper third of the stratum is lighter in color 

 and carries no large boulders. Above the blue gravel there is a remarkable stratum, varying from 

 three to ten feet in thickness, of the so-called " rotten boulders." These have evidently been ex- 

 posed to decomposing agencies, for, though still retaining their shape, they contain a great deal of 

 yellowish-red iron oxide, and are very easily broken up. The line between the rotten boulders 

 and the blue gravel frequently passes directly through a pebble or a boulder in such a way that 

 the upper half has to be reckoned with one stratum and the lower half with the other. Above 

 the rotten boulders there are generally, though not always, a few inches of a hard iron cement. 

 Above this comes the main mass of fine gravel and sand, reaching to the surface or to pipe-clay, 

 which in its turn is sometimes capped with basalt. The material of this stratum is almost exclu- 

 sively quartz. The pebbles are seldom as large as a hen's egg, and they are all exquisitely rounded 

 and smoothed, like the pebbles on a beach. The stratum varies in thickness in different parts of 

 the mine. In some places it is fully 200 feet thick. Upon this point, however, it is not easy to 

 get accurate data. I made the altitude of the top of the stratum in the high bank at the northern 

 base of the Sugar Loaf to be 1,377 feet ; its base was concealed from view. The color of this body 

 of gravel is not constant. The banks nearest the Sugar Loaf have a peculiar pink or a delicate 

 purple color, which is not so noticeable on the banks to the west and south. The cause of this 

 color is to be looked for in the presence of an unusual amount of rose quartz in the gravel, or to a 

 surface discoloration possibly originating from the decomposition of the overlying basalt. In 

 structure, also, this stratum of gravel is quite different from the great majority of banks seen in 

 other gravel-mining districts. It does not show, for instance, the regular stratification of the Mala- 

 kolf gravel, nor any approximate parallelism of flat, lenticular masses. On the contrary, it is 

 commonly and almost universally the case that limited portions of the gravel, homogeneous in them- 

 selves, are cut off abruptly by other portions, differing in color, in fineness, or in thickness, and in 





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which the lines of lamination make large angles with those of the first. 





