SIDNEY PAIGE will spend the summer in a study of the Homestake ore body in South Dakota.

F. C. SCHRADER, of the U. S. Geological Survey, spent June and July in field work on the mining districts of the Carson Sink quadrangle, Nevada.

A. C. SPENCER, assisted by G. C. Stevens, hydraulic engineer, recently investigated possible sources of additional water supply for the town of Strasburg, Va., at the foot of Massanutten Mountain.

JULIAN D. SEARS, of the U. S. Geological Survey, is continuing detailed study of the coal field at Gallup, N. Mex.

C. K. WENTWORTH, of the U. S. Geological Survey, assisted by J. B. Eby, is now engaged in field work on the coals of Wise County, Va.

C. E. SIEBENTHAL, of the U. S. Geological Survey, spent most of May and June visiting the zinc mines of Wisconsin, the lead mines of Missouri, and the fluorspar deposits of Illinois.

L. G. WESTGATE, of the U. S. Geological Survey, has sailed for Alaska and among other duties will investigate the recently discovered gold lodes of the Portland Canal region.

DAVID B. REGER, of Morgantown, W. Va., is on leave of absence from the West Virginia Geological Survey for the next four months and will devote that time to consulting work in petroleum and coal. He has just completed a month's trip to the prospective oil fields of Montana.

D. F. HIGGINS, geologist, expects to have his headquarters at Loveland, Colo., after September 1. He has just recovered from a serious attack of typhoid contracted during recent oil work in Egypt and Sinai.

The last twenty-five years have witnessed rapid strides in the development of the tin veins of the Bolivian cordillera. Once confined largely to the silver-rich ores of what is now the southern portion of the district, the range of known deposits has been steadily expanded until this distinct metallogenetic province extends over six degrees of latitude and embraces many veins of widely differing characteristics. The variety of types and relations progressively revealed has been closely paralleled by a

variety of genetic theories, each in turn founded upon more comprehensive information than was its predecessor.

Considering only the complex sulphide ores of silver, tin, and bismuth associated with igneous rocks, consolidated at no more than moderate depths, Stelzner1 regarded the Bolivian tin deposits, to which he applied the name Typus Potosi, as complete exceptions to the normal occurrences of tin with granite throughout the world, and attributed to them solely a hydrothermal origin.

He knew of the existence of predominant tin veins in the north and also of their association, geographically at least, with the granite core of the Cordillera Real. He did not know that such high temperature minerals as tourmaline and topaz were present, and he accepted and reaffirmed the antiquity of the granite relative to the age of ore deposition. Stelzner thereby fixed upon mining and geological opinion the impression that these tin veins differed fundamentally from previously known occurrences in lacking high temperature or "pneumatolitic' characteristics. A review of the literature reveals no evidence for the commonly repeated Paleozoic age of the granite of the region. Many of the invaded rocks are, no doubt, Silurian and Devonian sedimentaries, but, as we shall see, there are as good. or better lines of reasoning pointing to a much more recent origin for the granite as well as the elevation of the Bolivian cordillera. When the pronounced difference in the character of the cassiterite veins and the complex silver-tin veins compelled recognition in any tenable theory, Steinmann2 attempted to explain everything as centering about the latter, better-known type. The cassiterite, quartz, pyrite association was classed as an outlying facies of the mineralization, deposited at a greater distance from the source than the silver-bearing sulphide ores. In the light of present-day interpretation of the relations between temperature and ore deposition, this theory finds no adherents and will be shown to be contrary to facts in the course of this paper.

1 Stelzner, A. W., "Die Silber-Zinnerzlagerstätten Bolivias," Zeitschrift der deutschen geologischen Gesellschaft, vol. 49, pp. 51-142, 1897.

2 Steinmann, G., "Die Zinnerzlagerstätten Bolivias," Zeitschrift der deutschen geologischen Gesellschaft, vol. 59, Protokoll, pp. 7-9, 1907.

Later Rumbold3 and Armas emphasized the presence of abundant tourmaline in certain of the northern cassiterite veins and on the strength of this occurrence of a recognized high temperature gangue mineral, postulated a "pneumatolitic" origin for the deposits. Armas suggested that there had been a reopening of the veins accompanied by deposition of the silver minerals.

In 1912 Singewald pointed out the inadequacy of each previous theory to account for all of the facts, and advanced the first plausible explanation covering the two types of deposits. He accepted Steinmann's view of a common genetic source for all of the ore solutions, but wisely reversed the former interpretation of the relations between the cassiterite and the complex sulphide veins. In brief, he classifies the latter type of mineralization as that of the intermediate vein zone and the former as that of a slightly higher temperature facies. He does not consider that characteristic high temperature tin deposits occur in the district.

Seven years later Miller and Singewald restated the same theory in their book on South American mining districts. To quote the authors: "It is futile to attempt to establish an exact correspondence between these veins (Bolivian) and tin veins of the type represented by those of Saxony and Cornwall;" and "Inasmuch as the characteristics of the deep vein zone are but feebly developed in the most pronounced tin vein type, and the characteristics of the more pronounced silver vein type are those of deposits formed at moderate depths, and because further the texture of the genetically related igneous rocks is that of rocks which solidified at not greater than moderate depths, it appears 3 Rumbold, William R., "The Origin of the Bolivian Tin Deposits," EcoN. GEOL., vol. 4, pp. 321-364, 1909.

4 Armas, M., "Genesis of Bolivian Tin Deposits," Eng. and Min. Jour., vol. 29, pp. 311-314, 1911; "Etude sur l'étain et l'or en Bolivie et sur le genèse des dépots stannifères,” Annales des Mines, 10th Ser., vol. 20, pp. 149-230, 1911.

5 Singewald, Joseph T., Jr., "Some Genetic Relations of Tin Deposits," ECON. GEOL., vol. 7, pp. 263–279, 1912.

Miller, B. L., and Singewald, J. T., Jr., “The Mineral Deposits of South America," McGraw-Hill Book Co., New York, 1919, pp. 94-100.

certain that ore deposition took place under conditions of temperature and pressure prevailing at moderate depths." To many readers this statement of the case, though fitting many of the facts and inspiring conviction, must remain partially unsatisfactory in that such minerals as tourmaline, topaz, and apatite are assigned in abundance to the intermediate vein zone. Such exceptional occurrences constitute a serious obstacle to placing these deposits in their natural position in the genetic classification of mineral deposits, and in removing this obstacle it is hoped that an advance, however slight, will be made toward an orderly arrangement of our knowledge of ore deposition.

The author became interested in the Bolivian tin ores during some months spent in the Andes several years ago. Specimens. examined at that time and descriptions of the districts received from mining men of the region first suggested the outline of this thesis. Though an unexpectedly early return to the United States prevented a study of the deposits in the field, the problem was kept in mind and the re-statement of Singewald's early theory quoted in the foregoing paragraph has influenced the writer to publish this paper. Two suites of specimens studied recently in connection with ore dressing tests have supplied important genetic evidence, and a fine collection of tin ores presented to the Massachusetts Institute of Technology by Mr. Howland Bancroft has made it possible to broaden the scope of the work to include a general microscopic description of the ore minerals and their relations.

In short, the author proposes to prove that a normal and expected sequence of types follows a normal sequence of temperature conditions. The characteristics of the deep vein zone are very typically developed in the most pronounced tin vein type. This type is genetically connected with typical granitic rocks which probably solidified at considerable depth and ore deposition here took place under conditions characteristic of the high temperThe associated igneous rocks and the features of the mineralization conform closely to those of other high temperature tin veins throughout the world. The characteristics

ature zone.