eastward and joins Little Cholame Creek about 1,000 feet northeast of the center of sec. 31, T. 22 S., R. 14 E. Seeps of oil in granite are of course very unusual, though in the present case no unusual expla nation is necessary to account for the facts. The granite is a fault block which was crushed and jointed by movements along the San Andreas fault zone. Thus the normally impervious granite became sufficiently porous for oil to enter it from adjacent bodies of shale, in exa ly the same way that the oil which now seeps from the Paso Robles and Vaqueros entered those formations.

Economic possibilities.-The Parkfield syncline is the only fold in this area in which any great amount of the Santa Margarita (?) formation is present and in which it is overlain by a sufficient covering to retain any oil originally derived from the shale. About 2.500 feet of upper Miocene beds are included in the syncline, and oil might be expected to occur at their base on one or both flanks of the fold. This syncline could not safely be condemned as barren of oil by reason of the surface evidence, though the flanks of an isolated syncline of this type are not the most favorable structure imaginable.

The Tricounty well and the Table Mountain group of wells have given actual proof that only small quantities of oil are present, and as these wells penetrate the beds most likely to contain oil if it were present at all, further drilling upon this syncline would appear inadvisable.

Unfavorable results were also obtained by test wells in The Vallecitos, an area northwest of Coalinga, in which the structure is essentially similar to that of the Parkfield syncline. The tests of these two areas go far toward proving that isolated synclines of the oil-bearing series, surrounded by older rocks, will not prove productive in California. There are three factors which may account for the lack of oil. First, there may be an insufficient amount of shale present to have originally furnished any large quantity of oil. Second, the oil originally present could not be greatly concentrated at certain points, as migration could take place only along the comparatively short distance between the axis of the syncline and the outcrops of the oil sands on the flanks of the fold. In contrast are the productive San Joaquin Valley fields, where the oil in the productive folds may have come from areas far out in the valley syncline and also have migrated up the rise in anticlines. The third factor is the probable loss of oil at the outcrop of the oil sands. In an isolated syncline there is nothing to prevent the oil migrating up the dip and escaping at the outcrop.

1 Anderson, Robert, and Pack, R. W., Geology and oil resources of the west border of the San Joaquin Valley Dorth of Coalinga, Cal. U. S. Geol. Survey Bull., 603, pp. 167177, 1915.

Wells drilled for oil.-In the following descriptions of wells the numbers correspond to those on the geologic map of the Parkfield area (Pl. XXVIII):

At the localities numbered 1 and 2 remnants of rig timbers, etc., probably represent old holes drilled by Captain Barrett and associates before year 1900. The locations are unfavorable.1

Well No. 3, the Oakshade well, owned by the Monterey Oil Co., is said to have been drilled to a depth of 1,910 feet and to have encountered some oil and gas. This well is situated in the zone of complicated structure along the San Andreas fault. Such wells generally produce showings of oil but not enough to make them successful.

Well No. 4 indicates the location of the Middle Ridge Oil Co.'s derrick. No drilling has been done here and there is little chance of obtaining oil at this locality.

Well No. 5, the Tricounty well, is said to have reached a depth of 4,160 feet and found showings of oil at several depths. The well starts in upper Miocene sandstone and reaches the underlying Santa Margarita (?) formation near the bottom. Although there was some water trouble, which might account partly for the failure to obtain much oil, this well furnishes a pretty fair test of the west flank of the Parkfield syncline.

Well No. 6, the Parkfield Syndicate well, was drilled with a portable rig to a depth of 500 feet and got a small amount of gas. It was drilled entirely in the Vaqueros sandstone.

Well No. 7, the Miller or Raymond well, was the first of the Table Mountain group of wells, as drilling is said to have been started in 1888. The drill is said to have reached a depth between 640 and 800 feet without passing through any oil or tar sands. Sulphur water is now flowing from the casing. Of the wells of the Table Mountain Oil Co., Nos. 8 and 9 are 270 and 600 feet deep, respectively. They are said to be in oil or tar sand for most of this depth. Wells Nos. 10, 11, and 12 are shallow, ranging from 100 to 200 feet in depth; they are now abandoned, and when the writer visited them water and a small quantity of oil stood within 25 to 50 feet of the top of the casing in each well. The Table Mountain wells are said to have produced a few barrels of oil per day at the time of their completion. The oil doubtless originated in the Santa Margarita (?) formation and migrated up along the fault which lies east of these wells, saturating the sands for a few hundred feet out from the fault. The base of the upper Miocene sandstone also probably contains some oil which has migrated directly up from the underlying shale, in addition to that which came up along the fault plane.

1 Notes on the location and depths of these and other old wells may be found in California State Min. Bur. Bull 19, p. 145, 1901.

21544°-19-Bull. 691-17

Well No. 13, sometimes called the Livermore well, was drilled by the Future Success Oil Co. in 1913-14. It reached a depth of 1,810 feet, mostly in Cretaceous shale, but also apparently passing through intrusive sheets of serpentine and possibly reaching Franciscan rocks near the bottom. This well is separated from those on the west by a fault, and the light-gravity oil, showings of which are said to have been encountered, originated in the Cretaceous shale. Wells drilled in the dark Cretaceous shale generally get showings of a lightgravity oil but only a very small quantity.

Well No. 14, the Dominion well, was drilled in 1912 and is said to be between 150 and 300 feet deep. It is located in an area of serpentine slide, near the contact between the serpentine and the Cretaceous rocks. The bailer dump (consisting of the drill cuttings) appears to be made up entirely of serpentine, but some Cretaceous shale may have been encountered. The location is unfavorable.

THE SANTO TOMAS CANNEL COAL, WEBB COUNTY, TEXAS.

By GEORGE H. ASHLEY.

INTRODUCTION.

The cannel coal of Santo Tomas (sahn'toe toe-mahss'), Webb County, Tex., is of unique interest, both economically and geologically. It is of economic interest because it is probably the largest body of cannel coal of bituminous rank in the United States, if not in the world. It is of geologic interest because it is not the high-moisture, soft brown lignite which is characteristic of coal beds of the same age in the central and eastern parts of the State, but a low-moisture coal almost as hard as anthracite, resisting weathering like an ordinary bituminous coal and sold on Government contracts under a guaranty of 12,500 British thermal units on "dry coal." As a cannel coal it has special interest because on distillation such coal yields a much larger proportion of oil at a low temperature or of gas at a high temperature than ordinary bituminous coal, and at present and probably for some time in the future these products may be in great demand in the growing chemical industry of the United States. Recent tests by the Bureau of Mines have shown that although cannel coal yields a large percentage of oil when distilled at a low temperature, yet this oil contains so large a percentage of unsaturated hydrocarbons as to be unavailable for the manufacture of benzol and toluol products, which are in large demand at this time for the manufacture of high explosives. Most deposits of cannel coal are too small to justify the erection near by of a distillation plant of any considerable size for their utilization, but the amount of coal already in sight in this district warrants its investigation by those interested in chemical industries. (See p. 266.) A by-product of mining here is the large amount of clay that has to be removed in the mining operations. The clay already removed forms very large dumps (see Pl. XXXII) and lies ready for use in the manufacture any clay products for which it may be suited. It will probably not be found suitable for high-grade fire brick, but rather for vitrified ware, sewer pipe, draintile, and possibly paving brick.

of

This coal, which has been extensively mined for many years, is now used on eight or more railroads and in manufacturing plants

and has a growing use as a household fuel. Before the Mexican revolution it was much used by Mexican railroads and industries, supplying most of the territory that uses Laredo as a gateway. Its main drawbacks are the great cost of mining, due to thinness of beds and poor roof, and its high ash and sulphur content.

The coal is of Eocene (early Tertiary) age and, according to Vaughan, belongs in the Claiborne group, in contrast with lignite beds farther north, which are of Wilcox age. It lies nearly flat and almost undisturbed by horizontal pressure or mountain-building forces. The existence in the coal of fairly close-set joint faces, however, suggests that there has been some slight pressure in this area, which, in combination with the effect of dry climate, may account for the low percentage of moisture and corresponding high rank of the coal. The low moisture may result from the cannel character of the coal, as discussed beyond.

LOCATION.

The Santo Tomas coal field, as shown in figure 37, lies on the Rio Grande between Laredo and Eagle Pass, not far from Laredo. The boundaries of the field have not yet been determined, so that its possible extent is known only so far as to include the mines and prospects shown on the map. (See Pl. XXIX.) (See Pl. XXIX.) The developed area lies close to the Rio Grande, from 23 to 27 miles above Laredo, with which it is connected by the Rio Grande & Eagle Pass Railway. From Laredo the International & Great Northern Railway runs north to San Antonio and west of south 150 miles to Monterrey, Mexico; and the Texas-Mexican Railway runs east to Corpus Christi.

GENERAL PHYSICAL FEATURES OF THE REGION.

This coal field lies within the Rio Grande Plain, a part of the Gulf Coastal Plain, which is prolonged up the Rio Grande. This vast plain, here inclined southeastward at a rate of about 3 feet to the mile (see Pl. XXX, B), has been dissected by the streams, which now lie considerably below its level. The larger stream valleys are bordered by series of terraces. Near the Rio Grande this plain is eroded into an irregular escarpment by the smaller drainagewaysthe "breaks" of the Rio Grande. In places the plain reaches almost or quite to the river, but in general it is separated by terraces ranging from a fraction of a mile to several miles in width. Plate XXX, A, shows a view from such a terrace, about 75 feet above the river. This terrace is nearly destitute of vegetation in the dry season but is capable of cultivation by irrigation. Large areas of this terrace near Laredo are now irrigated, the water being derived from the Rio Grande by pumping and distributed by pipes and ditches. Plans are being considered for the irrigation of several thousand acres