stone and dolomite concurred, giving rise to the erroneous impression that the dolomite was favorable and the gray lime unfavorable for ore. The Bonanza main flat fault, however, is so insignificant as to be almost overlooked. It is an unusually even break, occurs along a smooth bedding plane, and contains only

FIG. 8. Plan of part of 500 level, Jumbo Mine showing relation of the “big chalcocite body" to the Flat fault, and the shapes and sizes of ore bodies disclosed on the level.

about one inch of compact gouge. Nevertheless, it appears to have been effective in terminating the downward extension of the main Bonanza vein from the fifth level to the surface. From the fifth level downward other small flat faults appear to have done the same thing. Nearly all of the irregular replacements, the so-called "flat ore," away from the main Bonanza vein, terminate downward against small flat faults, locally named "bedding planes," because they occur along bedding planes, and the evidence of movement is not pronounced. (See Fig. 12.)

Relation to Folds. At the Bonanza Mine, dips and strikes taken along the greenstone contact indicate a slight synclinal folding or else a monoclinal fold. The fold pitches parallel to the greenstone contact and the trough coincides with the ore zone. Underground exposures of the greenstone demonstrate the same thing. It may be considered simply as a downward warping of the inclined top surface of the greenstone and overlying limestone (Fig. 12). The ore appears to be connected with this fold

ing and it will be further discussed under the "Origin of Fractures."

Relation of Mineralization to Faults.-The larger faults already described apparently have no relation to the ore but there are in the mines numerous faults mostly of small displacement which have an important relation to it.

Where the pre-mineral faults or fault zones intersect the N.E. mineralized fissures there are local enlargements of the veins; the more brecciated the fault zone the greater is the ore enlargement, and where the intersection is nearly at right angles, the enlargement is greater than where acute. The enlargements are due to the opportunity afforded the solutions to be diverted from the main fissures into the cross faults, bringing about their filling and replacement. Some pre-mineral faults, like the flat faults, have acted as barriers to the further extension of mineralization. Post-mineral faults, or those faults which at least have postmineral movement along them, have displaced ore bodies.

Shapes and Sizes of Ore Bodies.-In a general way, the veins may be considered as extremely thin wedges gradually tapering upward toward their apex, and their bases resting on an inclined flat fault or bedding plane. The length of the inclined wedges along their base is many times greater than their height from base to apex. The height of most of the veins has been determined, but lengthwise along their base they are still being followed by inclined shafts (Fig. 6). In detail the veins are extremely irregular and depart from the wedge-like shape. The height varies from place to place along the incline; the width along the base varies still more, and also may change greatly from the base toward the apex, perhaps being wider midway up from the base than at the base itself, so that a cross-section at any one point may not show a simple wedge-like shape.

The average height of the main Bonanza vein from the base to the apex, measured normal to the incline, is about 210 feet in the upper levels and 150 on the lower levels. It has been followed for a distance of about 1,900 feet, measured along its base, and the width varies from 2 to 50 feet.

The main Jumbo vein, exclusive of its enlargement at the flat fault, averages about 360 feet in height, from 2 to 60 feet in width, and has been followed down its base for 1,500 feet. The great variation of width in both veins is due to locally greater replacement of the walls at particular places, intersections with fault zones, or junctions with other veins.

The lesser veins in both mines are, in general, similar to the ones described above, but not all bottom on the same flat fault, and their dimensions are not as great as in the main veins. The lesser veins appear to die out along their pitch, as do the main veins toward their apices, and where one dies out other parallel ones have frequently been found to start. Other veins occur in the Bonanza Mine formed by the replacement along vertical or highly inclined fissures which are abruptly terminated above and below by bedding planes. They are simply cracks across thick limestone beds (Fig. 12).

The big Jumbo ore body formed at the intersection of the main vein and the flat fault extends down the incline for a distance of 530 feet and has a maximum height of 100 feet, measured normal to its base. Its base is abruptly truncated against the flat fault and the shape may be seen from part of the plan of the 500 level shown in Fig. 8.

Most

The numerous irregular replacement bodies unconnected with the main vein in the Bonanza Mine are all limited on their bottoms by bedding planes or small flat faults. The tops of many of them are also limited the same way so that they extend as a series of more or less connected bodies up and down the dip of the strata and enclosed within certain beds of limestone. of them have a greater width than height, but in a few the dimensions are reversed. Up to the present they have been developed from the surface down to the 600 level. Their shapes are diagrammatically represented in Fig. 9. Other replacement bodies occur in both mines, but notably in the Bonanza Mine, as isolated blebs or bunches of chalcocite up to 15 feet in diameter, apparently unconnected with fissures or other ore bodies.

Relation to Surface. The moderately pitching ore zone of the

Bonanza Mine has been intersected by the steeply rising surface to form an ore cropping which is a cross-section from bottom to top, almost normal to the base of the zone. Thus the cropping displays the behavior of the veins in vertical extent, affording a cross-section of the wedge, with their bottoms spreading out on a bedding plane and tapering to their apex. (See Fig. 5.)

The relation of the present surface to the ore bodies is purely accidental and their exposure is due to the rapid surface erosion

FIG. 9. Diagram to show shapes of three types of irregular replacement bodies and their relation to the bedding planes.

which revealed them. It is a coincidence that, at the Bonanza, erosion had progressed to the point where two parallel veins and the mineralized ground between them formed an ore body much greater in extent than its continuation underground. Had erosion been less deep the great Bonanza cropping might not have been exposed, had it been deeper the big ore body might have been worn away. In the Jumbo the reverse is true; the cropping

although strong gives little hint of the great ore body that lies beneath. The vagaries of erosion were such that it exposed on the surface the weakest part of the vein, but had it extended deeper much of the great ore body might have been dissipated by the processes of mechanical disintegration. The surface croppings, therefore, give but little indication of what might lie beneath, nor does the absence of cropping preclude the possibility of ore below, for "blind" veins have been discovered underground. Hence the advisability of careful study of surface fractures.

Distribution of Ore Bodies.

Each mine contains one main vein and several lesser ones, the number being greater in the Jumbo than in the Bonanza. The lesser veins of the Jumbo are discontinuous vertically and horizontally, so that on different levels the number varies (Fig. 10).

FIG. 10.

Plan of part of 200 level, Jumbo Mine, to show shapes and distri

bution of veins.

Most of them are splits from the main vein that may coalesce with it again or fade out. On the 100 level are 4 separate veins : on the 200 level, 5; on the 300, 3; on the 400, 2; on the 500, 2; on the 600, 4 or 5, while on the 800 and 900 levels as yet only one vein is being worked on each, and these are not the same one. With the exception of the 900 vein all are contained within a narrow zone, and in the upper levels the mineralized walls of the