FROM FRENCH CORRAL TO NORTH SAN JUAN. 



387 



in the French Corral mines, the prevailing bed-rock is slaty or schistose in appearance, but it is 

 not such a well-defined slate as is seen in Svveetland Creek, or at the upper end of American Hill. 

 Further study will be necessary to make out its true relations. 



Buckeye Hill I did not visit ; the rock is said to be slate, as would naturally be expected from 

 what is known of the rock above and below. In Sweetland Creek, not far below the stage road 

 and near the outlet of the mines at Manzanita Hill, there is a lino of junction between the slate 



and the granitic rocks. 



At Manzanita Hill the bed-rock is a gneissoid granite. 



It is softer and 



more scaly on the west side than it is on the east. At xlmerican Hill the bed-rock exposed is 

 granite, excepting in part at the upper end of the claim, opposite San Juan. Here there is to be 

 seen again the junction of granite and slate with an intervening thin layer of a soft, slaty, or 



gneissoid character. The slates are line-grained, easily cleavable, and have a glistening surface, 

 which makes them resemble to some extent ordinary roofing slate, but they are not strong enough 

 to be used for that purpose. They stand with a vertical or very high westerly dip. The line of 

 junction has a nearly north (magnetic) course, and along this line a tunnel was formerly driven 

 with great ease. The position of this line of junction I have given approximately upon the map, 

 but I did not trace it beyond the limits of the mines. The bed-rock at San Juan is also a granite, 

 but of a variety which weathers very rapidly indeed, and crumbles to sand. About the middle ot 

 the San Juan mines the exposed bed-rock presented for a distance of live or six hundred feet along 

 the channel a very peculiar appearance. From a little distance it looked like irregular heaps ot a 

 bluish-gray clay, with smoothly rounded surfaces, arranged like little knolls or hummocks. The 

 surface looked as smooth and clean as if it had been brushed by Chinamen. A closer inspection 

 showed that the rock had crumbled to a sand, so little compacted together that I could remove it 

 with the finger alone to a, depth of live or six inches. Below this, the rock became gradually 

 harder, and appeared to be a true granite. Near the eastern end of the mines the rock in its 

 decomposition showed a decidedly marked concentric structure. 



At Manzanita Hill the granite was worn into large, broad, smooth surfaces, quite unlike the 

 wear previously described as seen at Smartsville. There was also a high point, like an island, near 

 the centre of the workings. There were some long furrows, and an occasional crevice. At Ameri- 

 can Hill the most marked irregularity in the surface of the bed-rock was the occurrence of " pot- 

 holes." One of these was from six to seven feet across at the top, about three feet at the bottom, 

 and twelve feet in depth. Its surface was smoothly polished. 



It may here be added that the rim-rook at the outlet of the French Corral mines is on all 

 sides higher than the lowest rock a few rods to the east. Prospecting shafts and tunnels have 

 been made in many places in the hope of finding some deeper trough. The conclusion reached is 

 that there is a bowl-like ending to the channel at French Corral, as at Timbuctoo. Probably the 

 continuation of the old channel was in the direction of the ravine, which is now followed by the 

 stage road from French Corral to Bridgeport. 



There is observable throughout this district the usual division of the gravel into a red above and 

 a blue below, but the thickness of the blue stratum is by no means uniform. The term " channel" 

 is sometimes used in different senses, here as well as elsewhere, either to include the whole body 

 of gravel from rim to rim, or only that deeper portion which "pays" the best, whatever be the 

 color of the gravel or the conformation of the bed-rock. For example, the " deep channel " is said 

 to be in some places only from 125 to 175 feet in width, while blue gravel is met with over a 

 width of twelve or fifteen hundred feet. As a rule, however, the blue gravel, is confined within 

 narrower limits than the red, and there is usually a narrow trough, OT "gutter," which can be 

 traced pretty well where the bedrock is exposed. At Manzanita Hill the bed-rock is nearly level 

 for a width of 900 feet, while at American Hill and at San Juan there is a deep trough nearly 

 midway between the rims from which the bed-rock rises on both sides. The average width of 



hannel, from rim to rim, between French Corral and San Juan is certainly not less than one thou- 

 sand feet, and probably as much as twelve hundred. I found it impossible to get accurate data 

 upon this point without going to the expense of new surveys. At Manzanita Hill the rims have 



