FOREST CITY AND VICINITY. 433 
further in lava cement. This tunnel is near Tyler’s Diggings, described in a previous page, and 
not far from the Nigger Tent House, but I cannot give its precise position. The course of the 
tunnel is N. 10° E. (magnetic). 
B. Forest City anp VICINITY. 
The portion of the ridge included under this head is comprised within the limits of the Moun- 
tain House on the west and the head of Wolf Creek, at American Hill, on the east. There is no 
map of the whole of this region to which I can refer, excepting that of Sierra County, but I have 
compiled one by transcribing the principal features of the public surveys of T. 19 N., R. 10 E., 
and a part of township 18 of the same range. (See Plate Q.) The ridge is cut up into several 
nearly parallel ridges by the deep caiions of Wolf, Kanaka, Oregon, and Rock creeks, the first 
three discharging their waters into the Middle, and the last into the North Fork of the Yuba. 
The steep northern slope is also deeply cut by Woodruff Creek, Slug Caiion, Slate, Castle, and 
Secret ravines, and Jim Crow Caiion, following the order from west to east. 
At the Mountain House, as has already been stated, the lava capping of the ridge has been com- 
pletely eroded. The separate, finger-like, longitudinal ridges have a capping of tufa or other 
voleanic material upon their higher portions, and above Forest City, near the head sources of 
Kanaka, Oregon, and Rock creeks there is a broad, nearly flat plateau of volcanic origin, which 
extends without interruption, though varying much in width, beyond American Hill, the easterly 
limit of my survey and observation in this vicinity. I have indicated upon the map the general 
position of the voleanic flow, but cannot vouch for the accuracy of its boundary in detail. There 
had been a heavy fall of snow just prior to the time I was in this region, and I was not able to 
reach all the points of interest. 
The most of my observations upon this volcanic material were made in the immediate vicinity 
of Forest City and Bald Mountain, or along the trails between Forest City and Minnesota on the 
south, or City of Six on the north. 
In its lower portion the volcanic material belongs, as a rule, to what is known as “ cement” ; 
that is, an aggregate of boulders or pebbles of greater or less size, though usually small, showing a 
rounded and smooth surface, and held together by an earthy cement as a rather loose and easily 
broken mass. The rounding of the pebbles may be due in some cases to the wear to which they 
were subjected in their journey from higher altitudes, or it may be explained by the existence of a 
concentric structure in the pebbles themselves. It is not uncommon to find lava pebbles with 
a marked tendency to break up, like an onion, into successive coats or shells. Together with 
the boulders, which are undoubtedly to be classed with volcanic tufa, and which resemble those 
met with on the San Juan ridge, there are also to be seen upon the surface, if not within the 
cement, boulders of granite and of several varieties of porphyritic and hornblendic rock, which do 
not so obviously belong to the volcanic series. They are sometimes very striking in appearance, 
and attract attention by their oddity, being quite different from the prevailing type. Mr. Wadsworth 
describes them as andesitic in character. I do not know of the existence of any similar rock in place. 
Higher up in the voleanic mass there appears to be a persistent stratum, which may best be 
described as a mud conglomerate. I observed it near the base of the Bald Mountain bluff, and on 
the ridge between Forest City and Alleghany. A few rolled quartz pebbles are occasionally seen 
on the surface, and, as I was told by Mr. Wallis, the superintendent of the Bald Mountain gravel 
mine, similar pebbles are to be found in considerable quantity at a perennial spring cf water, which 
breaks out under the lava of the bluff. 
The highest volcanic stratum is a compact, fine-grained, rather light-colored basalt.* This 
* This rock is thus described by Mr. Wadsworth: ‘A light gray, somewhat laminated rock, which shows 
fluidal structure, which has been the cause of the lamination. It is a surface specimen, evidently obtained from 
near the original surface of the lava. In the thin section the fluidal structure appears under the microscope to be 
strongly marked, and the rock is seen to be composed principally of basaltic plagioclase, innumerable granules, 
grains, and little crystals of augite, magnetite, olivine, and opacite. The base has been altered to a dirty brown 
fibrous mass, and considerable secondary feldspar formed.” 
