GRAVEL AND VOLCANIC FORMATIONS: AMADOR COUNTY. 



127 



extremely difficult to refer any portion of it to its original source in the High 

 Sierra. The region near Q. Ranch, a few miles northwest of lone City, 

 shows a number of these isolated mound-like outliers of volcanic sedimentary 

 deposits* One of these elevations, called Pratt's Hill, four or five miles west 

 of lone City, is made up of a series of strata of clays of different colors and 

 textures. There are live of these beds, which seem to have been formed, in 

 large part, from the trituration of the debris of volcanic materials, resting hori- 

 zontally on the bed-rock, which is here a hard, compact, fine-grained silicious 

 slate, somewhat ferruginous in character. On the clay beds is a deposit of 

 coarse gravel, made up almost exclusively of well-rolled pebbles of lava. This 

 is succeeded by a fine gravel cemented by volcanic sand, and capping the 

 whole a solid (andesitic?) lava. 



The volcanic deposits cover a large amount of surface in the vicinity of 

 Jackson, which town is situated upon the slates, but is surrounded by an 

 almost continuous line of elevations capped with volcanic materials. Large 

 areas to the south and southwest of the town have been washed off and 

 found rich in gold. North of Jackson and extending along the south side 



of Sutter Creek is a continuous 



ridge of lava, known as Humbug Hill. 



The thickness of the detrital deposits here is about 250 feet; of this, about 

 200 feet consist of finely stratified materials, and the upper fifty of imper- 

 fectly rounded masses of lava (andesite ?) which grow larger towards the top 

 of the stratum. The thickness of the pay-streak is quite small, in places not 

 over one or two feet, and it is made up chiefly of pebbles and slightly rolled 

 fragments of quartz, including, however, some very large boulders of the 

 same. The channel is said to be about 125 feet in width. 



The " Butte," about four miles east of the town of Jackson, is an isolated 

 knob, from which a fine view of the adjacent region may be had. The sum- 

 mit is elevated about 1,200 feet above the town of Jackson, and 800 feet above 

 its base. At the base of the Butte, on the west side, an excavation made by 

 prospectors shows horizontal strata, of white clay, sand, and finely powdered 

 pumice-stone, the thickness of which could not be ascertained. Above this 

 apparently horizontally stratified and undisturbed sedimentary volcanic de- 

 posit is at least 500 feet in thickness of a hard eruptive material of a reddish 

 color, supposed to be basaltic in character.* This mass is distinctly lami- 

 nated or stratified, and the layers have a variable strike and inclination. 

 The prevailing direction, however, is northeast and southwest, and the dip, 

 which is very irregular, is in places as much as 50° or 60°. 



* The specimens obtained in this vicinity were all destroyed by fire. 



