incomplete oxidation usually takes place and this feature may explain the lack of complete oxidation in the Kennecott deposits. Such a condition of deep cutting on either side of a narrow ridge would tend to produce a deep water level within the ridge. It would also cause considerable motion of the groundwater downward to the nearby deep valleys and the flow would be more rapid along channels of easy movement, such as fractures, than in the inter-fracture areas. Such conditions of deep and relatively rapidly downward moving groundwater would also give rise to an irregular surface of the water table; lower where prominent fractures occur and higher in the inter-fracture areas. These features are exactly the conditions required to explain the circumstances, already described, of intense oxidation produced along fractures, the paucity of it in inter-fracture areas, and the great depth to which it has gone.

Under such conditions of topography, the water level can hardly be considered to have been the customary sea of slow moving or stagnant water, for the great head would propel it toward the adjacent deep valleys and the numerous fractures would facilitate and accelerate the downward movement toward a lower outlet. It might be likened to a bath-tub full of water, with water pouring in and also running out by drain pipes, so that there is a continuous, rapid, downward motion. Under such circumstances, oxygenated waters could travel downward in conduits far below the surface of the groundwater and produce deep oxidation.

Before the oxygenated waters had completed their work of oxidizing the sulphides they were arrested by the frigid climate of the Glacial period, and frozen to form a "fossil groundwater" which has remained to the present. The mine workings disclose that all fractures, vugs, cavities, and pores are partly or completely filled by ice. It was the last mineral to form and encloses oxidation products. But the two lowest levels of the Jumbo Mine and the lowest one of the Bonanza are completely free from ice or water and the second lowest level of the Bonanza is almost free from it. They are dry and dusty with open frac

tures and vugs and show considerable oxidation. The mine. workings have thus penetrated a zone of frozen water, which in pre-glacial times was liquid. The bottom of the zone is wavy and exhibits prominent peaks and sags. Where the rock is not greatly fractured the ice zone extends lower and forms sags, but in prominent fractures like the Azure fault the peaks run well up into the frozen zone.

The interesting question arises as to whether this was originally the bottom of the groundwater, such as has been found in many deep mines. The abundant oxidation on the bottom levels, and the shape of the cross-sections of the ridge rather precludes this, however. If the mine temperatures were to be raised now, these openings would unquestionably become filled by water. The condition may be conceived as being due to frost encroaching downward and freezing the waters, but before the waters occupying openings in the lower levels could become frozen and fixed, they drained downward and left open cavities which could not again be filled by the overlying frozen waters.

Age of Oxidation.

The foregoing paragraphs indicate that present oxidation on the surface is negligible, while none can take place underground. It antedates, therefore, the freezing of the waters underground. This freezing must have taken place shortly after the inauguration of the low temperatures of the glacial period, so that all of the oxidation must have occurred in pre-glacial time. This conclusion is further supported by the fragments of oxidized ore sealed within the ice of the Glacier ore body. They must have fallen in during the building up of the glacier in the Glacial period and since they cannot have become oxidized since then, they must have been oxidized before that time. Their presence in the glacier also indicates that the ore body outcropped in the Glacial period and was not first exposed by the erosion of the Glacial period. All of the oxidation is, therefore, pre-glacial.

Oxidation of the ore deposits probably started during the development of the mature surface upon which the volcanics were

extruded. (See p. 15.) The water level was probably shallow at that time and migrated downward very slowly, giving those conditions under which thorough oxidation usually takes place. The greater part of the oxidation at present exposed, however, most probably took place during the time of the rapid erosion immediately preceding the Glacial period. Then, the topography and erosion were such as to give rise to the incomplete and deep oxidation so characteristic of the Kennecott deposits. The process of oxidation was vigorously operating until it was arrested by the Glacial period.

THEORETICAL CONSIDERATIONS.

Origin of Fissures.

Suggested Hypothesis.-The origin of the fissures of such unusual behavior has been a puzzle since they were first studied; various hypotheses were tested and discarded in the successive seasons. The usual theories of origin of fissures do not apply, because wedge-like fissures of little or no faulting, starting from the bottom and dying out upward, are decidedly different from ordinary fissures.

One hypothesis conceived in the spring of 1917 and tested in the field the following season appears to be borne out by the field relations, namely: that the fissures were formed by the stretching of the limestone beds due to a slight synclinal folding, or possibly a monoclinal fold, superimposed upon the flank of the major anticline, with its axis parallel to the dip of the limestone strata and normal to their strike. Careful mapping by structure contours underground and on the surface at the Bonanza Mine demonstrated that such a syncline or possibly a monocline exists, and that the ore bodies are located in its trough. The lack of sufficient exposures at the Jumbo prevented a similar test being made there. Such a synclinal fold would cause a stretching of the lower beds or the lower part of any group of beds, and a compression of the upper beds or the upper part of any group of beds. In brittle rocks such as limestone, under light load, the tension would be relieved by fissuring at or near the place where

the tensile stress was greatest. The fissures thus formed would trend along the line of greatest stress, or along the trough of the syncline or monocline; they would start at the bottom of some strata and extend upward to a point where the tensile stress diminished sufficiently that it no longer needed relief by rupture; the fissures would be strongest and most numerous at the bottom and taper upwards to the point where they died out; they would also probably start from the bottoms of more than one bed. Beds, or groups of beds acting as a unit, would, if of considerable thickness, contain fissures of considerable dimensions; if of small thickness, fissures of small dimensions. Fissures formed in this manner would start (and thus be abruptly terminated downward) at any prominent parting in the beds, such as strongly developed bedding planes or flat faults. More or less parallel fissures would be formed, staggered with each other, longitudi

FIG. 12. Diagrammatic vertical section to illustrate probable method of formation of wedge-shaped fissures (black) near axis of slight synclinal fold. Beds dip away from reader. Bottom of fissures follow same dip as beds; the tops approximately so. Greatest dimensions of fissures is along dip.

nally and vertically. The above conceptions are diagrammatically shown in Fig. 12, which is also a diagrammatic cross-section of the ore zone.

This hypothesis would adequately account for the inverted, wedge-like shape of the veins; their downward termination at the flat faults, and the shorter fissures extending across a few beds only. The upward termination throughout their length, at an approximately uniform distance above the base, is readily explained by the cessation of the rupturing due to lessened tension at a definite height from the bottom. The distribution of the mineralized fissures and the lack of appreciable faulting along them is in keeping with the theory, and their confinement to the brittle limestone is to be expected. In short, the suggested hypothesis appears to explain readily all the features of the fissures previously discussed.

A comparison with some of the structural features of the Mississippi Valley lead and zinc deposits is enlightening. Discussing the origin of the fractures in the Wisconsin area, Chamberlain states,27 "where the beds were (originally) bent downward, as in the case of the ore-bearing basins, the horizontal force would cause them to bend more deeply, and these would likewise be most fractured along the bed of the depression. In each case the fractures would gape mainly on the outer curve of the strata, i.e., on the under side of the sag." Again Chamberlain states, 28 "that they were formed as here indicated by the necessary fracturing of the bent strata on the outer side, is, I think, placed essentially beyond question by a consideration of the significant circumstances, (1) that they lie in stratigraphic depressions, (2) that both their upper and lower flats habitually sag, (3) that they lie almost wholly in the basal beds of the stratum bent..." On the following page he concludes. "that the strata should furnish a suitable receptacle for the ore at a given horizon and not above or below, is clearly indicative of some general physical cause." The most notable difference in the structural relations of the two is that in the Wisconsin area the synclinal depressions are in the form of basins, whereas at Kennecott it is a long trough. This difference would account for 27 Chamberlain, T. C., " Geology of Wisconsin," vol. IV., 1879, p. 484. 28 Op. cit., p. 486.