1 This paper embodies the results of field and laboratory studies of the
Kennecott deposits carried on at various times during the last five years.
The authors were together at Kennecott in the field season of 1915, when the
work was initiated in cooperation with the Secondary Enrichment Investi-
gation. Field work was continued by the first named author during the four
following seasons in his capacity of consulting geologist to the Kennecott
Copper Corporation, while most of the detailed microscopic work was done
in the laboratory by the other, in connection with research for the Secondary
Enrichment Investigation.

General. The copper deposits of the famous Bonanza and
Jumbo Mines at Kennecott, Alaska, are unique for the character
of their ores and their purity and size. Their occurrence pre-
sents many peculiar and interesting features, and the origin of
the great masses of chalcocite has long been a puzzle. Their
deciphering involves the origin of fractures unusual in form; of
a peculiar kind of primary mineralization unparalleled in other
deposits; of a source of metals not customarily considered, as
well as agents of transportation but seldom referred to.
No con-
clusions can be reached without carefully weighing the primary
or secondary origin of the chalcocite, and much of interest is
gained regarding the distribution of oxidation and groundwater.
It is with these problems that the paper deals, and the facts and
conclusions presented in the following pages are the result of a
study extending over a period of five seasons, during which time
the development of the ore bodies has been carefully followed
and their details accurately mapped.

Location. The Bonanza and Jumbo Mines, the most impor-

tant of the mining properties of the Kennecott Copper Corp., are situated at Kennecott, Alaska. (See Fig. 1.) This town lies about 200 miles northeast of the port of Cordova, on Prince William Sound, with which it is connected by the Copper River and Northwestern Railroad, a road which winds around glaciers and is famous for the difficulties of its construction. The Port of

FIG. 1. Index map to show location of Cordova, Copper River and Kennecott.

Cordova is navigable the year around and the railroad maintains continuous service with Kennecott, suffering only slight interruption, inevitable with its location.

The mines are one mile apart and each one is at a distance of three miles from the town of Kennecott2 and 4,000 feet above it, at an elevation of about 6,000 feet. They are connected with the mill in the town by aërial tramways.

2 The U. S. Geol. Surv. terminology gives the spelling as Kennicott but the post office and company name are spelt as Kennecott. As the latter is common usage it is followed in this paper.

Acknowledgments.—We take pleasure in acknowledging the courtesies and help rendered by the staff at Kennecott in 1915, and the aid received from the geologic maps prepared by W. E. Dunkle and H. D. Smith. The first named writer further records with appreciation the benefit received in the seasons since 1915 from discussions with E. T. Stannard, Wm. Douglas, H. D. Smith, D. C. Hoyt, and D. D. Irwin, and the helpfulness of their information concerning the detailed development of the ore bodies. Acknowledgment is due to Mr. Stephen Birch, president of the company, for placing all facilities at our disposal and for his permission to publish this material.

The painstaking areal work by the members of the United States Geological Survey facilitated our observations and deductions, and we acknowledge more than the mere sources of the information recorded to the individual members in the footTo Professor L. C. Graton we especially desire to record our grateful thanks for his many discussions and sympathetic criticisms.

notes.

TOPOGRAPHY.

The region in the vicinity of the mines is one of pronounced relief with high, steep-sided mountains, rugged in detail, rising from Kennecott Valley (Plate I., 4). The valley itself is occupied by the Kennecott glacier, which has a width of about three miles at the town and extends northwestward about twenty-four miles to its gathering grounds on Mt. Blackburn, 16,140 feet, and Mt. Regal, about 14,000 (Plate I., B).

The mountain sides on the Kennecott edge of the glacier rise steeply to the sharp divide of Kennecott spur at an elevation of about 7,000 feet. The Spur juts southward from the main Wrangel Mountains and is limited on the west by Kennecott Valley and on the east by McCarthy Creek, and rises from an elevation of 2,000 feet at the glacier and 2,650 feet at McCarthy Creek, to nearly 7,000 feet (Fig. 3). The top of the divide where occupied by the Chitistone limestone is serrated into perpendicular pinnacles and hoodoo forms of the shattered brittle

a. Kennecott spur and town of Kennecott, showing topography and location of mines (+).

b. Kennecott glacier. Mt. Blackburn in background and greenstone-limestone contact at right.

c. "Hoodoo" topography developed on fractured limestone adjacent to Bonanza Mine.