AGE AND ORIGIN OF THE BASALTS. 



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platforms, rising high above the surrounding plains, have an average 

 width from east to west of perhaps one hundred miles. While there are 

 small outliers of basalt on the south side of the Santa Cruz River, this 

 region may very properly be described as commencing on the north side 

 of that stream and extending northward throughout and beyond the limits 

 of the region under discussion. 



To the westward of this basalt-covered district, between it and the base 

 of the Andes, is the Western Plains Region, with an average breadth of 

 perhaps thirty miles. Its general features are not unlike those of the 

 western portion of the Magellanian Region, of which it is in fact but a 

 northern extension. Over considerable areas its surface is characterized 

 by similar lakes, drumlins, and other morainic features. These indicate 

 that formerly much of its area was buried beneath snow and ice. At 

 present, though still entirely unoccupied, it is, for pastoral purposes, by 

 far the most valuable of the plains regions of this part of Patagonia. 



Age and Origin of the Basalts. — Darwin was wont to consider the 

 basalt and lava beds of the plains of Patagonia as derived from the 

 Andes. He has cited them as extreme examples of long distance and 

 submarine flows over the but slightly inclined bottom of a shallow sea. 

 Had Darwin's opportunities for studying the interior of Patagonia been 

 greater, I do not believe he would have arrived at this conclusion. It 

 should be remembered, in this connection, that his only opportunity of 

 studying the interior and its lava fields, was during the exploration of 

 the Santa Cruz River, which he and Captain Fitzroy ascended to within 

 some thirty miles of Lake Argentino, without, however, discovering that 

 magnificent body of water. And even on this one protracted expedition 

 into the interior, it is evident, from his journal, that his observations were 

 chiefly limited to the high bluffs on both sides of the valley. There is, I 

 believe, in his narrative of this expedition, only one recorded instance 

 where he left the river and climbed to the summit of the bluff above the 

 valley. Nor is this to be wondered at, considering the fatiguing nature 

 of the work, to which each day they were subjected, in tracking three 

 heavy whaleboats against the swift current of the stream, and to the labor 

 of which, he says, they all gave a hand. A somewhat careful study and 

 extensive observation of the lava fields convinced me that the materials 

 composing them are of a varying age, and that, while some of them may 

 have been derived from the Andes, for the most part their origin may 



