arching of the major anticline. In such an arching the uppermost beds would be subjected to a stress which would be relieved by breaking, or by sliding upward over the lower beds, in the same way that the outer sheets in a pile of papers arched upward have to slide upward over the lower ones to allow of the arching. If such be the case the flat faults would be reverse.

PHYSIOGRAPHIC DEVELOPMENT.

The physiographic development of the region bears upon the relations of the land surfaces and previous water levels to the origin and superficial alteration of the ores. The record is anything but complete and the presentation of it here is, therefore, rather sketchy.

Pre-Jurassic Development.-As the base of the greenstone in this section is not known, there is no pre-greenstone record. The top of the greenstone is as regular a surface as any sedimentary layer. The even distribution of the amygdules from top to bottom of the individual beds is suggested by Moffit and Capps1o as one reason for believing them to have been of submarine origin. The relative coarseness of grain of the flows and the entire absence of ropy structure such as occurs in basic subaërial flows also suggests submarine origin. Further, the absolutely unweathered surface of the greenstone as shown in the deeper underground exposures indicates that it was not subject to weathering by the air. It would thus appear that the upper surface of the greenstone was not a land one.

The uninterrupted sedimentation of the Chitistone and McCarthy formations indicates no land area until the completion of the McCarthy shale at the close of the Triassic. Then uplift and tilting of the sea bottom initiated erosional processes which cut down into the McCarthy and Chitistone formations and, in places, into the greenstone. The resulting land surface appears to have been one of maturity with appreciable relief, consisting of relatively flat interstream areas and fairly deep, though gently sloped valleys. The boulders of the basal conglomerate, with a thick10 Op. cit., p. 60.

ness ranging up to 200 feet,11 in the succeeding Kennecott formation suggests that the land from which they were derived had considerable relief.

Jurassic to Tertiary Volcanics.-The land area next recorded is that developed on the Kennecott formation, and antedating the Tertiary volcanics which form the higher parts of the Wrangell Mountains to the north of the Kennecott area. This surface as seen from Bonanza Peak truncates the older beds and appears to be one of slight relief. It probably extended over the vicinity of the mines, although higher than the present surface. It is not known whether the Tertiary volcanics which lie on this surface ever covered the region in the vicinity of the mines, but they do lie on the Kennecott spur about 9 miles north of Bonanza Mine.

The groundwater level during this period of the development of a surface of slight relief had probably a gently undulating surface corresponding with the topography, a comparatively shallow depth, and with a probable slow downward migration. Its relation to oxidation will be discussed later.

Tertiary Volcanics to Glacial Period.-Deformation of the Tertiary volcanics indicates disturbance subsequent to their extrusion, which, with the erosion that followed, appears to have outlined the main features of the present topography.

A rugged topography seems to have been formed before the start of the glacial period and directed the main lines of glaciation. The master streams flowed in deep valleys and the main tributaries were established. Erosion, apparently, was rapid and great relief was produced. In fact, the main topographic features must have been very similar to those of to-day, except for the accentuation and changes produced by glacial and post-glacial erosion. Moffit and Capps12 believe that the uplift of the present mountainous areas acted slowly over a long period of time and was later than the Tertiary coal formations of Alaska.

During the carving of the master valleys, and the main tributaries, such as Kennecott Valley and McCarthy Creek, Kennecott 11 Op. cit., p. 38.

12 Moffit, F. H., and Capps, S. R., Bull. U. S. Geol. Surv. 448, 1911, p. 74.

Spur, lying between these closely spaced parallel tributaries, was formed, and must have undergone active erosion. The cutting of these two deep, closely spaced, tributaries must have lowered rapidly the groundwater level in the narrow Kennecott Ridge lying between them. (See Fig. 3.)

Glaciation and the Present Surface. The land carved by stream erosion was profoundly modified by glaciation, which in this country still persists in lessened form. The master valleys and major tributaries were filled by great glaciers, which broadened, straightened, and deepened them; spurs were truncated, lower ridges overridden, hanging valleys formed, and islands of resistant rock left projecting from the floors of the valleys. All those features characteristic of glaciation were impressed upon the country.

Moffit and Capps13 estimate the amount of glacial deepening to be between 1,000 and 1,500 feet and the top of the Kennecott Glacier to have stood about 3,000 feet higher than it is to-day. This means (and examination of the ground substantiates it) that the ore deposits, lying approximately 4,000 feet above the Kennecott Glacier, were unaffected by widespread ice sheets or large valley glaciers. The upper parts of Kennecott Spur were, however, vigorously attacked by local mountain glaciation, as is attested by the numerous cirques, many of which intersected and produce sharp arretes and steep peaks. Bonanza Peak still supports five short glaciers, one of which is even now sapping the outcrop of the Bonanza ore bodies and enriching its moraine with copper minerals. Elsewhere along the spur, rock glaciers are frequent, and small glaciers are active. The buildings of the Jumbo Mine are built upon the edge of one such small glacier and are constantly moving, while a power line which crosses it show an annual movement in the center.

In addition to the local glacial erosion, frost action played an important part in lowering the general level of the spur and sculpturing the details of the present rugged topography already described. How much of the upper parts of the ore bodies or their overlying cover was thus carried away cannot be told.

13 Op. cit., p. 44.

Post-glacial stream erosion in the vicinity of Kennecott is almost negligible, because the post-glacial period is yet to come. Here and there shallow notches have been cut by the streams into frozen till or soft rocks. Frost action, however, is active and is now an important agent of erosion. Due to it many of the slopes are mantled by great talus slides resting at the angle of repose and extending in a direction hundreds or even thousands of feet, downward from the base of the steep upper cliffs. The talus. slopes are continually creeping downward, due chiefly to the action of snowslides in winters, or of melting snow in spring, and are as continually being renewed.

No evidence of more than one period of glaciation has been observed in the Kennecott district. If an earlier glaciation did take place, its record has been obliterated by the intense. erosion of the recent glaciers. It is probable that here as in other parts of Alaska these were periods of glacial advance and retreat11 and the present period may be one of temporary glacial retreat.

The effect of the glacial period on the ore bodies was to bring about a rapid erosion of their upper parts and to arrest all oxidation or chemical changes within the ore bodies by freezing the waters that would bring about such changes.

ORE DEPOSITS.

General.

The ore bodies at the Bonanza and Jumbo Mines are of the same type, the same character of ore, and exhibit similar behavior. They both occur in the same formations and although a mile apart, were formed by the same processes. In detail and size they differ considerably. Other deposits of the same character of ore and very similar in type, in the immediate district, are the Erie and Mother Lode Mines, both of which have, as yet, made only small ore shipments. The Jumbo and Bonanza have

been the great shipping mines.

14 Such a condition is considered by Capps to have occurred in the White River district. Capps, S. R., Jour. Geol., vol. 23, p. 748, 1911, and U. S. Geol. Surv. Bull. 630, p. 63, 1916.

Character of Ore.

The ore is worked for its copper content, but an appreciable amount of silver adds to its value. No other valuable metals are extracted from the ore or occur in more than microscopic amounts. It is largely sulphide with considerable carbonate of copper scattered promiscuously through it. Occasionally the latter, consisting chiefly of malachite and minor azurite, constitute the chief part of the ore in particular places. Of the copper produced in the last few years, about 25 per cent. has been derived from carbonates. The carbonates have resulted entirely from the oxidation of the sulphide.

The sulphide is almost wholly chalcocite15 and the great masses and purity of this mineral are one of the striking features of the deposits. The gangue consists entirely of limestone or dolomitic limestone country rock. In places, in and adjacent to the ore, the limestone is recrystallized into white and mottled calcite. Rhombs of pure white calcite are frequent.

Shelter analyses of ore shipments, (1) first quarter of 1915 and (2) last quarter of 1918, show that it contains:

It will thus be seen that the ore bodies were formed by the deposition of copper sulphides, with practically no other minerals or gangue matter. The discussion of this feature will be taken up under "Theoretical Conditions."

Types of Deposits.

General. In the Kennecott and surrounding districts, copper deposits occur in both limestone and greenstone. Those in the 15 Other sulphides, in almost negligible quantity, occur. For the amount and description of these see "Mineralogy.”

16 This silica is probably obtained from impurities of the lower limestone country rock.

17 11.68 per cent. of this copper is in the form of copper carbonate.