The basalt is coarse to fine grained, usually black or dark gray, and is often scoriaceous. It contains plagioclase, augite, and olivine, and abundant magnetite in a micro-crystalline or granular base. Near the foot of the range the basalt has overflowed the beds of tufa, which latter, it is said, contains from a trace to as high as four ounces of silver per ton. As far as I have any knowledge of it, no prospecting has been done in these tufa beds, but the fact that silver exists there at all would lead one to believe that under proper conditions ore bodies of great value may occur.
At the base of the mountains, at an altitude of 1,800 feet, is a dry lake, which drains a large area of country. In this basin water has been obtained by sinking a well 80 feet in depth. Though the well is located near the edge of the basin, there is little doubt that sufficient water can be obtained in this basin for milling purposes.
The principal vein in the district lies along the north side of the main range, and is known as
THE IMPERIAL LODE.
This great vein, the outcrop of which may be seen for 20 miles, is a “fissure” in every sense of the word, though not a simple one, for it has numerous divergent branches of considerable size. The main fissure, however, is strong and constant, and outcrops boldly for nearly 8,000 feet. It varies in width from 4 to 18 feet. It is everywhere well defined and often shows a banded structure.
The great fissure strikes north 70° west, and dips 65° to 70° to the south. It occurs in the quartz porphyry, which at contact with the vein is usually much decomposed and often shattered and crushed, probably owing partly to the intrusion of a large dike of the red felsitic rock, a tongue of which has cut across the vein about 3,500 feet from its west end. This dike follows the vein for some distance on the hanging wall side coming from the east, gradually nearing the vein until it finally reaches the fissure, cutting the vein in two. Farther westward it again appears on the hanging wall side, showing itself at intervals to the extreme western end of the vein, which comes to an abrupt termination. This felsitic dike is but one of a number that occur in the immediate vicinity.
THE IMPERIAL LODE LAVA BEDS DISTRICT
SAN BERNARDINO CO. CAL.
VEIN CUT BY INTRUSIVE DYKE OF FELSITE
Since the formation of the Imperial lode there has been considerable movement within the vein itself. Slips are numerous, the slickensides showing plainly. The fault planes, as far as observed, are confined within the limits of the vein; however, at no place, excepting where the dike intersects it, did I notice any lateral displacement.
Usually the vein is distinctly separated from the inclosing rock, a clay selvage marking the line of the fault plane on either side. In some instances, however, where a brecciated condition of the quartz porphyry is found in contact with the vein, the ore has been deposited to some extent in this broken mass, and in such cases the line of demarkation is not at all plain. These occurrences, together with the branching spurs, seem to indicate perfectly the character of the vein, which before the intrusion of the felsitic dike was more simple than we now find it. My conception of the Imperial vein is that a great fissure formed in this mountain range; that at the time this disruption occurred the hanging wall side of the fault slipped downward, causing a further fissuring and crushing of the rocks on that side of the fault. It is a notable fact that all the branches or spurs of this vein occur in the hanging wall country and are directly connected with the main fissure. This idea is still further substantiated by the additional fact that all, or nearly all, the crushing and grinding adjacent to the fault plane has occurred on the hanging wall side.