THE VOLCANIC CAPPING OF THE GRAVEL. 



103 



5. The Volcanic Capping of the Gravel ; its Thickness and General Character. 



On the road, a short distance above Independence Hill, may be seen a small outcrop of " white 

 lava," -which material is very rare in this vicinity. It underlies the bouldery cement which forms 

 the crest of the ridge. Half a mile northwest of Independence Hill the capping of volcanic debris, 

 on the crest of the ridge, is probably 400 feet thick. On the northern slope of the first Sugar 

 Loaf Hill, just southwest of Iowa Hill, a tunnel has been driven, at a point northwest of the Colfax 

 road, N. 15° W. into the gravel. The crest of tins hill is covered with an aggregation of earthy 

 matter with fragments and boulders of all sizes, up to many tons in weight, and only partially 

 weather or water worn. The thickness of this capping is uncertain ; it probably does not exceed 

 one hundred feet, and may be less than fifty. 



There are in the Lebanon Tunnel, on the northeast side of New York Canon, many very large 

 boulders of very hard and compact volcanic rock, thoroughly smoothed and rounded by water; as 

 much so, indeed, as any of the pebbles in the ordinary metamorphic and quartzoso gravel. 



In the ridge between Iowa Hill and Damascus the volcanic conglomerate is frequently overlain 



by heavy masses of breccia, the total thickness of volcanic matter often ranging from 300 to 500 



feet. The bed-rock at the Hog's Bank, between Damascus and Secret Hill, is said to be from GOO 



to 800 feet below the crest of the ridge ; or, in other words, that is the supposed thickness, in that 



region, of the deposits of detrital and volcanic material. 



On either side of Sailor's Canon, four or five miles east of Canada Hill, and nearly parallel with 



it, are two small canons, and the basin to which they belong is approximately semi-circular in form 

 and has a radius of three or four miles. The whole of the ridge around this basin is capped with 

 volcanic debris of all sorts, beds of ash alternating with masses of well-rounded and water-worn 

 conglomerate, but with no solid lava. The depth of this capping seems to vary from a, few hundred 

 to a thousand feet in thickness. From here a sharp high peak, on the crest of the Sierra, bears 

 N. 37°,$. (magnetic), and from this peak around to the north the whole summit as far as visible, 

 that is through an arc of some 15° or 20°, consists of volcanic rocks horizontally stratified, and the 

 depth of this deposit must be very great, probably from 1,500 to 2,000 feet. 



North of Dead wood, along the western brow of the ridge fronting the East Fork of El 

 Dorado Canon, the gravel, which is a few feet in thickness only, is overlain by volcanic depos- 

 its in horizontal strata, and which are made up of alternations of sandy and clayey materials with 

 heavier masses of bonldery accumulations. This volcanic capping acquires a thickness of several 

 hundred feet, as we go back towards the crest of the ridge. At the Eeed Mine, near Dead wood, 

 the whole mass of the gray cement contains large quantities of magnetic iron in line grains ; but the 



white pumice-like spots which occur in the cement appear to be particularly rich in this mineral. 



The volcanic " cement " occasionally contains fragments of quartz included in it, showing that it 

 has been transported from a distance, as at Hornby's Tunnel near Dead wood, and other localities. 

 On the crest of the ridge, just above Deadwood, two or three huge volcanic boulders were seen ; 

 these were smoothly rounded, and one of them was estimated to weigh not less than twelve or 



fifteen tons. 



In the sides of the ditch running from Deep Canon to Last Chance, the volcanic formation is 

 seen to consist chiefly of only partially rounded pebbles and boulders, it being rather a breccia 

 than a conglomerate. The rocky fragments of which it consists are of all sizes, from small pebbles 

 tip to boulders and blocks of five or six tons, or even more in weight. 



On the gravel in Jones's Hill, northwest of Georgetown, there lies a heavy stratum of micaceous 

 volcanic sand ; then, a layer of gravel, about four feet thick, containing a little gold, and above that 

 the ordinary bouldery volcanic cement. 



Immediately north and northwest of Colfax the volcanic formations are seen in the crest of a 

 ridge rising to 400 or 500 feet above the town. There is at least 200 feet in thickness of volcanic 

 material here, and the crest is narrow and sharp and covered with boulders, some of which will 

 Weigh two or three tons. These boulders are rounded ; but not so much as the volcanic fragments 







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