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.
[Illustration: _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.
Occasionally, in the narrower portions of the vein, a banded structure
indicates the probability that the ore now fills what was at one time an
open crevice, which slowly filled with ore by precipitation from the
mineral waters passing along the fault plane.
At one point on the course of the vein, where it is joined by one of the
branching spurs, the felsitic dike has intersected the smaller vein. The
occurrence is plainly seen in a cut made at this place, where rich ore
was found.
[Illustration: _THE FELSITE DYKE
CUTTING A BRANCHING VEIN
IMPERIAL LODE_]
It will be noticed that both the main vein and the spur occur in the
quartz porphyry, and that the red felsite cuts the smaller vein. The
displacement on the surface along the strike of the vein is nearly 40
feet. This does not show in the cross-section.
Since the formation of the Imperial vein the mountains have suffered
great erosion; the highest point along the croppings rises fully 800
feet above the neighboring cañons.