







I 



446 



SUPPLEMENTARY INVESTIGATIONS IN THE GRAVEL REGION. 





At the southern base of the lava ridge is the com men cement of the exposed gravel deposit, 

 which extends by way of Secret Diggings to Barnard's, being cut off only at Rabbit Creek and 

 Clark's Ravine. The gravel apparently extends backwards underneath the lava, and a bank simi- 

 lar in character would naturally be looked for on the northern slope of the ridge, near Little Grass 

 Valley. I saw no signs of any bank, however, upon that side, and I was told that the drifts, 

 which have been run under the ridge from the south, were cut off abruptly by a wall of gray lava, 

 and that all efforts to find the continuation of the channel in that direction had been unsuccessful. 

 It was not possible for me to examine the old workings, and, consequently, I cannot verify these 

 statements from personal observation. There is a theory held by some, that here was one of the 

 original fissures from which the volcanic material was ejected. I. am not ready to accept this 

 theory, though I see no reason to doubt the statements made relative to the cutting off of the 

 gravel. There is abundant additional evidence that this section of country has been the scene of 

 unusual disturbance since the deposition of the gravel. 



It was not easy to make out the character of the gravel at the northern extremity of the deposit 

 — at the base of the lava — on account of the heavy slides from the top which have taken place 

 since active mining operations were suspended. As nearly as I could estimate, there was a stratum 

 of fine white quartz gravel at the bottom, about fifty feet in thickness, upon which, and reaching 

 up to the base of the lava, was a stratum of equal or greater thickness of a yellowish or grayish 

 pipe-clay. From this bank downwards through the La Porte and Secret Diggings mines, for a dis- 

 tance of over two miles, the greater part of the deep gravel has been removed. There are still con- 

 siderable masses of gravel in places along the rims, which will ultimately be washed away ; and the 

 amount still remaining at Barnard's is also probably large, but I am not able to make any specific 

 report. The gravel in the banks which remain unwashed, and the boulders — 

 which have been left upon the bed-rock, are both almost exclusively either white or bluish quartz. 

 The gravel is fine, the pebbles seldom exceeding four or five inches in diameter, though some of 

 the larger boulders are as much as six or eight feet through. At Secret Die<>irio- s there is some 



large and small 



t>r> 



sand, and occasional bunches of pipe-clay are mixed with the gravel near the top. The boulders 

 seen upon the bed-rock other than quartz doubtless came from the surface, and are mostly of a vol- 

 canic character. Some of them contain porphyritically disseminated hornblende. The quartz is 

 well-washed and rounded, and it generally has a brilliant lustrous surface. This latter feature is 

 so characteristic of the gravel of the old channels in this vicinity that all gravel, however well 

 smoothed and rounded, which is dull in lustre or lacks clearness of color, is spoken of with con- 

 tempt as "bogus," "river wash," or "bastard gravel," and is said to be barren of gold. T saw 

 considerable " bogus gravel " upon the surface of the ground to the east of the main La Porte 

 channel. In the upper portions of the La Porte banks the gravel exhibits a well-marked, small- 

 lenticular stratification, or has, using the terms employed in Dana's Manual of Geology, a "sand- 

 drift" or a " flow-and-plunge " structure, in striking contrast with the more uniform stratification 

 of the banks at Malakoff and other points in Nevada County. 



The general course of the channel at La Porte is southeasterly, but at Secret Diggings there 

 is a long, sweeping curve to the southwest. The rini on the west is usually at a lower level 

 than that on the east, which is due in part to the erosive action of the western creek and ravine, 

 and in part to the disturbances whose effects are more noticeable on the east. For a part of 

 the distance there seem to be two parallel channels, the more easterly one running upon a 

 higher bench of bed-rock. To the north of the town of La Porte tin; bed-rock is quite un- 

 even, the channel being divided by island-like masses, which rise to heights of thirty feet or 

 more above the deeper portions. Below tin; town the channel is more regular. At Secret Dig- 

 gings, in the widest portion, there is a well-defined, gutter-like channel along the centre. The 

 width of the channel or of the gravel deposit is not uniform. 



The observations which I took to determine the grade of the channel are not in all respects 

 satisfactory. I made the altitude of the deep northern bed-rock, at the base of the lava ridge, to 

 be 5,077 feet. At the southeastern outlet of the La Porte mines, where the channel is supposed 





