the localites from which prospectors have reported "ledge tin" will be described under the headings "Brooks Mountain," "Ear Mountain," "Hot Springs," "Asses Ears," and two other localities worthy of investigation will be mentioned under the headings "Diomede Islands" and "Don River."

LOCALITIES WHERE LODE TIN HAS BEEN FOUND.

LOST RIVER.

Lost River enters Bering Sea at a point about 15 miles southeast of York, 25 miles west of Teller, a town on Port Clarence, and 10 miles northwest of Point Spencer, at the entrance to Port Clarence. A view of the valley of this river, taken from the coast, is shown on Pl. III. The river has a length of about 10 miles and drains the central part of the York Mountains. The mountains constitute a nearly circular area of rugged land forms, about 15 miles in diameter. The summits rise to a general level of about 2,500 feet, and, as noted, reach a culmination of 2,900 feet in Brooks Mountain, near the north side of the area, which is the highest point in the northern part of Seward Peninsula. Along the southern edge of this mountain mass there is a well-defined bench from one-half mile to 4 miles wide. This bench was cut from the rocks by wave action and then raised, but so unequally that at the mouth of Lost River it has an elevation of 600 feet, while eastward it gradually declines until at Port Clarence it is practically at sea level.

The writer has referred to this feature in a previous paper as the Cape York bench." It was produced during the same period of erosion as the York Plateau.

On the seaward side the Cape York bench is bounded by steep bluffs, which at places front directly on Bering Sea (see fig. 1) and at other points rise from a lower and younger bench nearly at sea level. This lower and newer plane is well developed from the mouth of Lost River eastward to Port Clarence, and has a width varying from onehalf mile to 3 or 4 miles. It is, in part, a rock bench similar to the Cape York bench, and, in part, a gravel-built coastal plane. Immediately north of Port Clarence the lower coastal plane is fringed by a wide lagoon, cut off from Port Clarence by a sand spit.

The York Mountains are generally devoid of the tundra vegetation which covers so much of the Seward Peninsula; and along Lost River, from the coast to the tin deposits, can be found an exceptionally good roadbed for this part of Alaska. For one traveling on foot it is as firm as an ordinary macadamized road, and owing to the ease with which the trip up the river is made the distances are likely to be underestimated by persons who have traveled in other parts of Seward

a Collier, A. J., A reconnaissance of the northwestern portion of Seward Peninsula, Alaska: Prof. Paper U. S. Geol. Survey No. 2, 1902, p. 37.

Bull. 229-04- -2

Peninsula. Lost River forks about 14 miles from the coast, one branch continuing in a nearly due north direction, while the other drains a country to the west that has not be examined by geologists.

BERIN G

Igneous dike

Granite

SEA

FIG. 3.-Sketch map of Lost River. Scale, inch=1 mile.

About 4 miles from the coast the north fork of Lost River divides. The eastern branch is Cassiterite Creek; the western, which is somewhat larger, rises about 3 miles to the north, in the slopes of Brooks Mountain.

20

The Lost River tin deposits are located on the east side of the north fork of Lost River. (See fig. 3.) The ore has been found on Cassiterite Creek and on another eastern tributary, known as Tin Creek, which enters Lost River about a mile below the mouth of Cassiterite Creek. The latter stream has a length of about 3 miles; its head is within 1 mile of Cassiterite Creek, and after flowing parallel with Cassiterite Creek for about 1 mile it turns westward and enters Lost River from a deep canyon cut in the limestone of the York Mountains. At its mouth Cassiterite Creek is about 100 feet above the sea. In the latter part of July, 1903, Lost River carried approximately 1,000 miner's inches of water.

The York Mountains, in which the Lost River Basin lies, are composed almost wholly of ash-gray limestone of Silurian age, the Port Clarence limestone. Along Lost River the limestone shows little general metamorphism, and as a rule dips at low angles. From the coast to Tin Creek the strata generally dip to the north, and unless there are faults, which were not detected, a thickness of over 5,000 feet of limestone must be exposed. Near the mouth of Lost River a section of these limestones lying nearly horizontal is exposed in a mountain, called by prospectors Saddleback, which has an elevation of more than 2,000 feet above sea level. Dikes of igneous rock cut this limestone at several places along Lost River, and a number of these were readily traced across the limestone by a growth of moss and other vegetation which formed over them, the limestone itself being utterly devoid of vegetation. Microscopic examination shows that these dikes are of rhyolitic nature.

On Tin Creek, which enters Lost River from the east about 4 miles from the coast, a large body of granite was found intruded in the limestone. This granite outcrop is believed to be nearly circular in outline and probably one-half mile in diameter. Around its margin the limestone was found to be considerably altered, and some small dikes of fine-grained pegmatite, probably apophyses from the main mass, were found cutting the limestone, apparently parallel with the contact of the limestone and granite.

Under the microscope the granite from the main mass is found to consist essentially of quartz, biotite, hornblende, orthoclase and acidic. plagioclase feldspars with fluorspar, either accessory or secondary, and a few small grains of a mineral resembling zircon and believed to be cassiterite. Apparently the rock has been slightly crushed or sheared, producing streaks of fine-grained fragmental material of the same character as the original grains.

In Tin Creek, which flows for some distance along this contact, many bowlders and pebbles, some of considerable size, were found to contain minerals, which are the result of contact metamorphism.

The main tin-bearing ledge outcrops nearly half a mile north of this

granite boss. It is a white, porphyritic dike, cutting the Port Clarence limestone, and striking nearly east and west. It has been traced from Tin Creek westward across the mountain to Cassiterite Creek, a distance of about 1 mile, but has not been found beyond these streams in either direction. All of this rock has been more or less altered, so that it is practically a greisen having crystals of cassiterite disseminated through it. Specimens collected near Tin Creek appear, in the hand specimen, to be a white aplite or porphyry with some small spots and large patches of purple. Under the microscope many of the original minerals are seen to have been replaced by fluorite, to which the purple color is due. Pseudomorphs of fluorite take the place of most of the feldspar crystals and of some of the quartz grains. (See Pl. V.) In specimens which are still more altered, collected from the same dike, near Cassiterite Creek, probably very few of the original minerals remain. The rock here is found to consist of calcite, fluorite, lithia mica, and quartz, proportioned in the order named. The limestone, on the south side of the dike, is altered for several hundred feet, and contains many greenish minerals, among which epidote and garnet have been identified. The limestones north of this dike are reported to contain many small stringers of tin ore for several hundred feet. The ore obtained from the main ledge varies considerably in general appearance and character. Some of the weathered ore from the croppings is highly siliceous, and has the appearance of weathered, iron-stained vein quartz with small black cassiterite crystals disseminated through it, while other specimens show clearly their granitic origin and contain comparatively little vein quartz. In the ore of the latter type the cassiterite occurs both as disseminated crystals varying in size from that of a pin head to that of a walnut and as veinlets and irregular masses. (See Pl. VII, B.) The granitic ore consists principally of calcite, fluorite, quartz, and large crystals of lithia mica; and in addition to the cassiterite, tourmaline, topaz, pyrite, garnet, and galena were observed in small amounts. Quantitative analyses of the lithia mica present made by W. T. Schaller, of the United States Geological Survey, show that it has the composition of zinnwaldite. In the float of this dike large specimens of galena, wolframite, and some malachite were collected, and in the altered limestone near the contact some large specimens of garnet were obtained. The siliceous ore mentioned above, when examined with the hand lens, sometimes showed spangles of free gold. A sample of this ore assayed for gold and silver gave 0.36 ounce of gold per ton and a trace of silver. The piece assayed was a picked specimen, and not a commercial sample. Assays made for other parties are reported to show smaller amounts of gold in all The occurrence of so much gold associated with the cassiterite seems to be unusual in tin ores, and merits further investigation.

cases.

a Assay by E. E. Burlingame & Co., Denver, Colo.