Yol. 59.] GEOLOGY OF PATAGONIA. 165 



The hornblende-granite forms a link between the biotite-granite 

 and the quartz-mica-diorite, and, judging from the profusion in 

 which these three types occur together on the northern shore of the 

 lake, it may be safely assumed that they have been derived from a 

 common complex where one type merges into another. 



In view of the abundance of hypersthene in the sediments of 

 Eastern Patagonia, it is remarkable that no hypersthene-andesites nor 

 tufTs containing hypersthene-fragments were found in the moraines. 

 This leads to the conclusion that the irruptions of granite and 

 porphyry in this part of the Cordilleras have masked these rocks, 

 and that they took place after the deposition of the Santa Cruz Beds 

 and prior to the formation of the Tehuelche Pebble-Bed : in fact, 

 they were probably contemporaneous with the formation of the 

 tectonic valleys of the Cordilleras to be noted later. 



IV. The Basalt-Flows. 



The enormous extent of the basalt-flows was noted by Darwin, 

 who found the end of the mass 67 miles from the mouth of the 

 Rio Santa Cruz. 1 I saw them exposed in the valley of the Chico 

 de Santa Cruz and at many points nearer the Cordillera, from which 

 chain they are cut off abruptly by a longitudinal depression well 

 seen between Lake Buenos Aires and LakePueyrredon, and, according- 

 to Dr. Moreno, of tectonic origin. Another basaltic area is found in 

 the region of Lakes Colhuape and Musters. The traveller journeying 

 from Chubut to Colhuape first encounters these flows in the valley 

 of the Rio Chico de Chubut, some 15 miles from the outlet of that 

 river from Lago Colhuape. Around the lake the basalt, which over- 

 lies light-coloured sediments, baked at the junction, is carved into 

 curious prominences visible for a great distance. Masses of basalt 

 occur again in the valley of the Senguerr and at the junction of 

 the Rivers Senguerr and Mayo. Whether anyone has definitely 

 connected this basaltic area with that which lies to the south- 

 west, having its western limit under the Cordilleras, cannot be 

 ascertained ; but, seeing that this region was in earlier times the 

 seat of orogenic movements, the remains of an ancient mountain- 

 chain being found near Lake Musters, 2 it is not improbable that it 

 represents a distinct but contemporaneous outburst. 



The lavas of the other, and better-known, area were first seen by 

 me on the shores of Lake Buenos Aires. On the northern shore only 

 in one place was basalt found in situ, near and east of Colle Piramide. 

 At the point of entrance of the Rio Eenix into the lake another small 

 mass appears, beautifully glaciated, and to the eastward the plains 

 are dotted with basalt-erratics. On the southern shore, at a con- 

 siderable altitude above the lake, is a vast plateau of basalt. Sloping 

 gently eastward, it extends without a break as far as the valley in 

 which the Arroyo Gio flows, and southward as far as the gap leading 

 to Lake Pueyrredon. Beyond these breaks are a series of plateaux, 



1 ' Geol. Obs.' 2nd ed. (1876) pp. 381-85 & fig. 34 (p. 379). 



2 F. P. Moreno, Geogr. Journ. vol. xiv (1899) p. 367. 



