BETWEEN NORTH BLOOMFIELD AND EUREKA. 299 
D. Between Norra Bioomrietp AND EuREKA. 
Immediately above North Bloomfield the lava capping on the ridge between éhe South and the 
Middle Yuba spreads out to a width nearly double that which it has below. Its surface for several 
miles forms a nearly flat table, with a gradual and regular grade rising to the northeast. The 
grade amounts to about ninety feet to the mile between a point on the ridge near Bloomfield 
and the summit, westerly from Shand’s Ranch. The sloping sides of the volcanic stratum towards 
the streams are steep, though not precipitous. The slope to the southwest near the head of 
Humbug Caijion is also quite steep, and to the traveller approaching from the direction of Lake 
City the town of North Bloomfield seems to be built in a broad amphitheatre, with high walls 
nearly enclosing it on all sides, excepting towards the southwest, where Humbug Cafion has 
its outlet. 
That the mass of gravel which covers the surface near Malakoff and North Bloomfield ex- 
tends under the volcanic capping, there can be no doubt; but precisely where the deep channel 
lies, and whether or not the gravel is confined to any one single channel, are questions to which 
as yet no satisfactory answers can be given. There are deposits of gravel which have been 
worked by the hydraulic process both on the northern slope of the ridge, as at Woolsey Flat, 
and on the southern slope, at Relief Hill, which can be traced to the edge of the lava or even 
underneath it ; and there have been shafts sunk through the volcanic capping from which im- 
portant information as to the position and character of the underlying gravel has been obtained. 
In my description of this portion of the ridge I will first state what is known about the con- 
nection between the Bloomfield gravel and that of Woolsey Flat, and then take up the considera- 
tion of the gravel deposits on the northern slope of the ridge, reserving to the last the deposits 
on the southern side. 
The two points between North Bloomfield and Woolsey Flat at which the rock lying under the 
volcanic stratum has been reached are at the Derbec Shaft and the Watt Shaft.* The Derbec 
Shaft lies about a mile to the north of the town of North Bloomfield, near the stage-road leading 
to the Backbone House. Work was begun on this shaft on the Ist of September, 1877, and 
was prosecuted without material interruption until the 18th of July, 1878, on which date the 
deep bed-rock was reached. The following data in regard to the materials passed through in the 
sinking are taken from one of the reports of the Derbee Company. The first gravel was struck at 
a depth of 167 feet from the surface, beneath volcanic cement. The first pipe-clay was reached 
at a depth of 208 feet, and the first gold-bearing gravel at 271 feet. The blue gravel was met at 
the depth of 310 feet. This gravel has a thickness of 150 feet, deep bed-rock being struck at the 
depth of 460 feet below the surface. 
The altitude of the mouth of the shaft I made to be 3,813 feet above the level of the sea, which 
will give 3,353 feet as the altitude of the bed-rock at the bottom of the shaft. Between this point 
and the Prospect Shaft at Malakoff, referred to on page 396, the position of the bed-rock is not 
known, but there is a difference of level between the places of about 360 feet. The grade must 
be as much as 200 feet to the mile, or possibly more. 
From the bottom of the shaft prospect drifts were run in several directions with the object of 
finding and tracing the course of the channel, and of opening the mine for work. By the kindness 
of the superintendent at the shaft, Mr. C. M. Cox, I was allowed to take from the plans of the 
workings the data necessary to show the evidence upon which rests the prevailing belief as to 
the course of the channel. They show that its direction was almost exactly from north to south 
for a distance of 500 feet from the north end of a drift run nearly parallel with and a little to 
the west of the channel, and that from there it followed a course curving a little to the eastward 
for about 400 feet farther. From these data it will be seen that rising bed-rock is known to 
exist at three points on what may be taken as the eastern rim of the channel. I was admitted 
to the underground workings, and found the blue gravel to be well cemented at the points where 
* See General Gravel Map for the position of these points. 
