CRUISE OF STEAMER CORWIN IN THE ARCTIC OCEAN. 



137 



years, their mechanical excellence is such that they reflect the 

 sunbeams like glass, and attract the attention of every observer. 

 The most perfect of these shining pavements lie at an elevation 

 of about 7,000 to 8,000 feet above the level of the sea, where the 

 rock is close-grained, silicious granite, though small fading 

 patches may be found at from 3,000 to 5,000 feet elevation on 

 the driest and most enduring portions of vertical walls, where 

 there is protection from the drip and friction of water; also on 

 compact swelling bosses partially protected by a covering of 

 bowlders. 



On the north half of the Sierra the striated and polished sur- 

 faces are rarely found, not only because this portion of the chain 

 is lower, but on account of the surface rocks being chiefly porous 

 lavas subject to rapid waste. The moraines, also, though well 

 preserved on the south half of the range, seem to be nearly want- 

 ing over a considerable portion of the north half, but the mate- 

 rial of which they were composed is found in abundance, seat 

 tered and disintegrated, until its glacial origin is not obvious to 

 the unskilled observer. 



A similar blurred condition of the superficial records-obtains 

 throughout most of Oregon, Washington Territory, British Co- 

 lumbia, and Alaska, due in great part to the action of excessive 

 moisture. Even in Southeastern Alaska, where the most exten- 

 sive glaciers still exist, the more evanescent of the traces of 

 their former greater extension, though comparatively recent, 

 are more obscure than those of the ancient glaciers of California. 

 where the climate is drier and the rocks more resisting. We 

 are prepared, therefore, to find the finer lines of the glacial record 

 dim or obliterated altogether in the Arctic regions, where the 

 ground is mostly low and the action of frost moisture specially 

 destructive. 



The Aleutian chain of islands sweeps westward in a regular 

 curve nearly a thousand miles long from the Aliaslca Peninsula 

 toward Kamtchatka, nearly uniting the American and Asiatic 

 continents. A very short geological time ago, just before the coin- 

 ing on of the glacial winter, the union of the two continents was 

 probably complete. The entire chain appears to be simply a de- 

 graded portion of the North Pacific pre-Glacial coast mountains, 

 with its foot-hills and lowest portions of the connecting ridges 

 between the peaks a few feet under water, the submerged ridges 

 forming the passes between the islands as they exist today, 

 while the broad plain to the north of the chain is now covered 

 by the shallow waters of the Bering Sea. 



Now the evidence seems everywhere complete that this seg- 

 regating degradation has been effected almost wholly by glacial 

 action. Yet, strange to say, it is held by most observers who 

 have made brief visits to different portions of the chain that 

 each island is a distinct volcanic u'nheaval, but little changed 

 since the period of emergence from the sea, an impression made 

 no doubt by the volcanic character of most of the rocks, ancient 

 and recent, of which they are composed, and by the many extinct 

 or feebly active volcanoes occurring here and there along the 

 summits of the highest masses. But, on the contrary, all the 

 evidence we have seen goes to show that the amount of glacial 

 denudation these rocks have undergone is very great, so great 



Aj 



