,166 ME. J. B. SCRIVENOR ON THE [May I903, 



all basaltic, no doubt at one time continuous, but now broken up 

 by valleys. In one valley, between those of the Deseado and the 

 Charcamac (the latter a small stream flowing from the plateau), I 

 had an opportunity of examining the section laid bare, in height 

 about 80 feet, a measurement which only represents part of the 

 total thickness of the flows. The section showed three bands of 

 compact rock separated by vesicular basalt, which probably represent 

 three distinct streams of lava. In the dense basalt, olivine, augite, 

 and plagioelase were visible, with minute flakes of a mineral which 

 was probably weathered brown mica. Some of the plagioclase- 

 crystals were partially re-fused. The vesicular basalt also showed 

 the above minerals, with the exception of the mica. A rough 

 columnar structure was observable in places. 



In the valley of the Chico de Santa Cruz both sides show sections 

 of the same basalt-flow for a great distance; in fact, on the left bank, 

 to within 70 miles of Santa Cruz itself in a direct line. On the right 

 bank the flow does not extend so far, owing to the convergence of 

 the valley of the Chico with that of the Rio Santa Cruz. In the 

 former valley are four remarkable isolated masses of basalt : one, 

 Sierra Oveja, I did not pass close enough to speak of with authority, 

 but Mr. Hatcher states that it is igneous. 1 This is the first mass 

 seen when descending the valley from the Cordilleras. The others, 

 Sierra Ventana,Baleria Sud, and Baleria JSTorte, are composed entirely 

 of basalt similar to that described above. The most conspicuous is 

 Sierra Ventana, a mass between 200 and 300 feet high, forming one 

 of the rare landmarks in the dreary East Patagonian landscape. It 

 rises sharply to a point, suggesting by its form a lofty German 

 mediaeval castle, and has, close to the summit, a natural window- 

 like aperture, from which the hill takes its name. 



That the wide-spreading basalt-flows were produced by the usual 

 type of eruption, that of small vents along lines of fissure, there 

 can be little doubt. Dr. Moreno has recorded craters at several 

 localities under the foothills; I saw them on the plateau over- 

 looking Lake Buenos Aires ; but to the east evidence of their existence 

 is not definite, so that it is possible that the flows may have travelled 

 for considerable distances, and that the vents were restricted to 

 a line (or lines) of fissures immediately adjacent to the foothills of 

 the Cordilleras, with a north-and-south trend. Mr. Hatcher has 

 described Sierra Oveja and Sierra Ventana as volcanoes, 2 and stated 

 that the latter consists of vesicular and slaggy basalt. Of Sierra 

 Oveja I cannot speak, for the reason stated above ; but the con- 

 figuration of the valley at this point led me to conclude that Sierra 

 Ventana, like Baleria Sud and Baleria Norte, is but a remnant of 

 the plateau spared from the denuding powers of the river. 



A noteworthy point, with regard to the angle of inclination of 

 the flows, is mentioned by Darwin, 3 that of the abrupt rise in the 



1 Am. Journ. Sci. ser. 4, vol. iv (1897) p. 339. 



2 Ibid. pp. 339 & 350. 



3 ' Geol. Obs.' 2nd ed. (1876) p. 384. 



