Vol. 59.] GEOLOGY OF PATAGONIA. 17& 



Specimens of igneous rocks from the moraines under the Cordil- 

 leras comprise biotite-granite, hornblende-granite, quartz-mica- 

 diorite, gabbro, hornblende-picrite, quartz-porphyry, rhyolites, 

 trachytes, ophitic olivine-dolerite, olivine-basalt, and acid tuffs. 



If hypersthene-andesites exist, they have been masked by the 

 later acid igneous rocks, which were probably contempora- 

 neous with the formation of the transverse depressions of the 

 Cordilleras. 



The basalt-flows are of enormous extent, and came from vents 

 along lines of fissure trending north and south. The basalt 

 near Lakes Musters and Colhuape marks a distinct but con- 

 temporaneous area of eruption. 



The basalt is older than the transverse valleys of the Cordilleras, 

 and older than the Tehuelche Pebble-Bed. 



No single agency can account for the formation of the Tehuelche 

 Pebble-Bed. It is due to glacial, to fluviatile, and also to 

 marine action. 



Part of the porphyry-pebbles in the Tehuelche Pebble-Bed have 

 been derived from the masses of quartz-porphyry east of the 

 basalt-flows. 



Lake Buenos Aires, draining into the Pacific, lies in a tectonic 

 valley cutting through the Cordilleras. Its original volume was 

 much greater than at present, and its outlet was to the Atlantic. 



The disproportionate size of the valleys crossing the pampas to 

 the streams that they contain is owing to decrease of rainfall, 

 capture of tributaries, and to the share that possible marine 

 denudation played in their formation. 



An ideal case of glacial ponding was found at the head of the 

 great Canadon Salado. 



No evidence was found to show that the valleys crossing the 

 pampas are fault-valleys. The outliers of basalt in the valley 

 of the Bio Chico de Santa Cruz are direct evidence to the 

 contrary. 



My thanks are due to my friend, Mr. H. H. Thomas, M.A., F.G.S., 

 who pointed out the true nature of the hypersthene in the sandy 

 Patagonian Beds, and the similarity of the mineral constituents to 

 those of the volcanic dust of Martinique ; also to those who gave 

 me their assistance in South America, foremost among whom a^e 

 Senores Hauthal, Santiago Both, Dr. P. P. Moreno, and Seiior Waag; 

 also to Mr. Roger Campbell, whose generosity made pleasurable 

 my stay at Monte Leon ; and to many others whose hospitality I 

 enjoyed. 



EXPLANATION OF PLATE XIII. 



Sketch-map of Southern Patagonia, on the scale of about 100 miles to the inch, 

 partly based on that drawn up by Dr. F. P. Moreno, by kind permission of 

 the Secretary of the Boyal G-eographical Society. 



