domes much more than the coastal belt and may therefore be of that older age. The more considerable erosion that Palangana has undergone may, however, be due to the fact that it is more favorably situated for erosion than the domes of the coastal belt. It is apparently older than the plateau level of the top of the cuesta and the Reynosa does not seem to have been greatly affected by the doming. How much the Lagarto and Lapara Beds were affected is not known.

ORIGIN OF THE DOME.

The origin of salt domes is still a much disputed question. There is a general tendency among geologists who have studied salt domes to recognize that the salt core is intrusive. The mechanism of the intrusion is a disputed question and no satisfactory explanation of it has as yet been formulated. little exploration that has been done to the present, Palangana offers no new evidence on the origin of salt domes and the various theories that have been advanceds will not be discussed here.

903 RUSK AVE.,

HOUSTON, TEXAS.

With the

8 G. D. Harris, "Rock Salt in the State of Louisiana," Bull. No. 7. Geol. Surv. of La., 1907; E. T. Dumble, "Origin of the Texas Salt Domes," Bull. A. I. M. E., No. 142, 1918; E. L. De Golyer, "The Theory of the Volcanic Origin of Salt Domes," Bull. A. I. M. E., No. 19, 1918; G. S. Rogers, "Intrusive Origin of the Gulf-Coast Salt Domes," Ec. GEOL., Vol. XIII., No. 6, 1918; F. F. Hahn, "The Form of Salt Deposits," Ec. GEOL., Vol. VII., No. 2, 1912.

TUNGSTEN DEPOSITS OF BURMA AND THEIR

ORIGIN.

J. MORROW CAMPBELL.

INTRODUCTION.

Burma and the Malay Peninsula consist essentially of a series of parallel mountain ranges, the axes of which are slightly west of north and east of south. The valleys between them vary in type from deep rocky ravines to wide fertile plains, depending on local conditions.

These great ridges were produced by granite thrust upward in the molten condition which penetrated to only a limited extent the overlying sedimentary rocks. No evidence has yet been obtained by which we can fix definitely in what geological period these uplifts took place.

Denudation has removed much of the overlying sediments and exposed granite in many places. These exposures vary in area from a few acres to hundreds of square miles. They occur much more frequently in the narrower portions of the peninsula where rainfall is more abundant and where debris more readily reaches the sea than they do in the north. The average vertical amount of rock denuded in the south has been very great, the zones of low and intermediate temperature ores having been removed, copper, lead and zinc found in the north have disappeared and ores of high temperature type, tin and wolfram, are comparatively abundant.

We are concerned at present primarily with tungsten ores, their mode of occurrence and origin, but the latter part of the subject involves the consideration of accessory minerals, especially cassiterite.

ORE DEPOSITS.

The Tavoy District has yielded much the greater part of Burmese tungsten ore; for this reason and also because the mode of

occurrence there is more interesting than elsewhere, most attention will be given to it.

Rock Formations.-The greater part of the surface in the Tavoy District is occupied by an apparently single series of argillites, phyllites, quartzite and schist to which the name Mergui schist is applied. They are non-fossiliferous and probably of Silurian age. What appears to be a continuation of this series to the north in the Moulmein District is overlain unconformably by the Plateau Limestone, known to be the homotaxial equivalent of our Devonian.

Granite is found at the base of the Mergui schist, wherever exposed. It is highly acidic and very uniform in composition. Its components are abundant quartz, less abundant feldspar (orthoclase and microcline) and a little mica (biotite). Sometimes mica is practically absent. Hornblende is unknown, tourmaline has never been observed and the minor accessory minerals usually found in granite are conspicuous by their scarcity. In some exceptional cases the granite near its periphery is markedly porphyritic.

Metamorphism of the sediments in the vicinity of their contact with the granite is not usually strongly marked but mica and andalusite schists have been observed in several places. It is notable that where intense contact metamorphism occurs neither the granite nor the schist at the contact contain ore minerals though they may be found some hundreds of feet above. In every case in which the granite or the schist contains ore at the contact the latter appears to have been only very slightly metamorphosed and resembles the same rock a thousand feet above the contact.

Mode of Occurrence.-Tungsten ores occur in veins in granite near its periphery or in the overlying sediments from the contact to over 600 ft. and probably 1000 ft. or more vertically above it, or partly in both.

In granite they have been proved to occur, (1) As primary minerals, (2) in greisen, (3) in veins of quartz, quartz with mica, and pegmatite. Greisen is the result of alteration of part of the

wall of a fissure and is not ore-bearing unless the contents of the fissure are also.

In the "schist" tungsten ores occur in aplite dikes and quartz veins.

Typical ore-bearing pegmatite has not been observed to occur in sedimentary rocks, but it seems impossible to draw a hard and fast line between pegmatite and quartz veins, for the latter always contain some mica. Many veins in granite are partly or wholly filled by material consisting of quartz, mica, ore minerals (sometimes in great variety) but absolutely without feldspar, and to which no other name than pegmatite seems to be applicable.

Ore-bearing veins, whether in granite or sedimentary rocks, are usually narrow, rarely exceeding 5 ft. in width and generally much less, but are frequently numerous, especially in the schist. Their strike usually approximates the direction of the axis of the batholiths but great variations occur, some veins striking East and West and in other cases two series of parallel veins within the same area, differently oriented cross one another. In such cases the series striking more nearly North and South is older than that with easterly and westerly strike. Aplite dikes, whether orebearing or not, when occurring in ore-bearing sedimentaries are invariably older than adjacent quartz veins. With the exception of a very few long single ones, tungsten-bearing veins occur in groups parallel to one another in strictly circumscribed areas outside which no ore is found and no evidence of fissuring.

The great majority of wolfram mines in the Tavoy District are small patches, the length of which along the strike of the veins may be exceeded by the width. It is exceptional to find either dimension over 2,000 feet. Kadwe Mine, which has yielded hundreds of tons of wolfram, and at the top of which veins were so numerous that thousands of cubic yards of rock weathered in situ were remuneratively worked by ground sluicing, is less than 800 feet across.

The dip of the veins may be in either direction, it is seldom less than 60 degrees and generally not far from vertical.

The great bulk of Tavoy wolfram is won either directly from

quartz veins or from detrital material on hillsides below vein outcrops: very few true alluvial deposits in Burma are tungstenbearing. Both greisen and pegmatite are occasionally rich, but the total amount of ore they yield is small. Tungsten ores occurring as primary minerals in granite have so far proved of economic value in only a few cases. Tungsten-bearing aplite is of scientific interest only, for its tenor is low and the dikes usually

narrow.

The existence of wolfram as a primary mineral in granite and aplite has been questioned but the opinion of the writer has been endorsed by that of Mr. J. Coggin Brown, of The Geological Survey of India, after microscopic examination of thin slices of the rocks. Such granites have been observed to contain not only wolfram and cassiterite but scheelite, molybdenite and bismuthinite.

A feature that strikes one forcibly in connection with the smaller wolfram occurrences is the extraordinary number of them that are to be found on hill-tops. In one case there are four on one side of a stream and three on the other quite unconnected with one another and though the hills are less than 300 feet in height, in no case does the ore, rich on top, persist to the level of the stream below.

That most of the Tavoy veins are worthless within 300 feet below their outcrop is established and, though there are notable exceptions, we have no proof that payable ore persists in any case for a continuous vertical depth approaching a thousand feet.

The Mergui District, south of Tavoy, has yielded some wolfram but most of its veins are stanniferous and do not need any

comment.

The Moulmein District to the north of Tavoy shows several ridges of granite. Here the overlying sediments have undergone intense metamorphism, much gneiss having developed. Orebearing veins are common but they contain cassiterite almost exclusively, the gangue being usually tourmaline pegmatite. No ore is found in the granite itself.

Farther north, along the same strike, at a distance of about 200 miles up the Salween river, on its right bank, in the Karenni State