PATAGONIAN BASALTS 793 



thosite series of Kosenbusch belongs with the Arctic clan, but reasons 

 have been adduced elsewhere 37 to show that this conjecture is unjustified. 



Patagonian Basalts 



Scattered through Argentine Patagonia, from the Eio Negro to Tierra 

 del Fuego, are numerous areas of plateau basalts. 38 These form thick 

 horizontal sheets, covering the sedimentary rocks, and have a total area 

 of "many thousands of square miles." They would seem to have been 

 first noted by Darwin during the voyage of the Beagle, and he attributes 

 their origin to Andean volcanoes. Both Hauthal and Hatcher, however, 

 point out that they have originated from numerous fissures and (accord- 

 ing to Hatcher) many cones, and they must have been very fluid. Both 

 Suess and von Wolff reckon them among the plateau basalts. Their age 

 is uncertain, but they would seem to be late Tertiary and some of them 

 much more recent. 



These lavas are spoken of as basalts by all who have written about 

 them, but there are no petrographic descriptions of them and no analyses. 

 In order to try to obtain some information along these lines, I endeavored 

 through the kindness of Prof. C. H. Smyth, Jr., to obtain some of Hatch- 

 er's specimens. It seems, however, that no specimens of these basalts 

 collected by Hatcher are to be found in the collections at Princeton, and 

 apparently he brought none back with him. Professor Smyth, however, 

 found a few specimens of basalt obtained from Ameghino, which he very 

 kindly entrusted to me for study. These specimens are all small frag- 

 ments, none of them very fresh or compact, and without any definite 

 locality noted on the label. Unsatisfactory as they are, however, it was 

 thought that study of them might be of interest, in view of the almost 

 total absence of knowledge concerning these basalts, and I would ex- 

 press my thanks to Professor Smyth for his kindness in furnishing the 

 material. 



The specimen selected for study was what appeared to be the freshest 

 and was stated to have come from "la base des couches a Notostylops," 

 Patagonia. The rock, megascopically, is a dense, compact, aphyric, 

 slightly brownish black basalt. In thin section it shows the usual sub- 



37 Cf. H. S. Washington : The charnockite series of igneous rocks. Amer. Jour. Sci., 

 vol. 41, 1916, p. 338. 



38 Cf. Darwin : Geological Observations, 2d ed., 1891, pp. 381-384 ; J. B. Hatcher : 

 Amer. Jour. Sci. (4). vol. 4, 1897, p. 350; R. Hauthal: Peterm. Mitth., 1903, pp. 97-102, 

 map 9 ; J. B. Hatcher : Princeton Univ. Exped. to Patagonia, vol. i (narrative and geog- 

 raphy), 1903, passim, and especially pages 109, 218-221, and 287; Suess: Pace of the 

 earth, vol. iv, 1909. p. 485 ; F. von Wolff : Vulkanismus, vol. i, p. 432, 1914. Some of 

 the western areas are mapped by P. Quensel ; Bull. Geol. Inst. Upsala, vol. xi, 1911. 



LII — Bull. Geol. Soc. Am., Vol. 33, 1921 



