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 débris, 
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 this 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 Cafion, 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 quartzose 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 600 
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 Caiion, four or five miles east of Canada Hill, and nearly parallel with 
it, are two small cafions, 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 débris 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° S. (magnetic), and from this peak around to the north the whole summit as far as visible, 
that is through an are 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 Deadwood, along the western brow of the ridge fronting the East Fork of El 
Dorado Cafion, 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 bouldery accumulations. This volcanic capping acquires a thickness of several 
hundred feet, as we go back towards the crest of the ridge. At the Reed Mine, near Deadwood, 
the whole mass of the gray cement contains large quantities of magnetic iron in fine 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 Deadwood, 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 Caiion 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 
up 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 voleanic fragments 
