CRUISE OF STEAMER CORWIN IN THE ARCTIC OCEAN. 139 



set free by the action of frost, rains, and general weathering- agents, considerable quantities are 

 swept down in avalanches of snow where the inclination of the slopes is favorable to their action, 

 afld shaken down by earthquake shocks, while the glacier itself plays an important part in the 

 production of these superficial effects by undermining the cliffs from whence tin' fragments fall. 



But in all moraines bowlders and small dust particles may be recognized as not having been 

 thusderived from the weathered cliffs and dividing ridges projecting above the glaciers, but from the 

 rocks past which and over which the glaciers flow. The streams which drain glaciers are always 

 turbid with finely-ground mud particles worn off the bed rocks by a sliding motion, accompanied 

 by great pressure, giving rise to polished surfaces, and keeping up a waste that never for a 

 moment ceases while the glacier exists. 



Moreover, bowlders are found possessing characteristic that enable the observer to follow 

 their trails and discover the positions of the channels whence they came. Accordingly, an abrupt 

 transition is here discovered from the polished aud plain portions of the channels to the more 

 or less angular and fractured portions, showing that glaciers degrade the rocks over which they 

 pass in at least two different ways, by grinding them into mud, aud by crushing, breaking, and 

 splitting them into a coarse detritus of chips and bowlders, the forms aud sizes of which being in 

 great part determined by the divisional planes the rocks possess, and the intensity and direction 

 of application of the force brought to bear on them, while the quantity of this coarser material 

 remaining in the channels along the lines of dispersal and the probable rate of movement of the 

 glaciers that quarried aud transported it, form data from which some approximation to the rate 

 of this method of degradation may be reached. 



The amount of influence exerted ou the Aleutian region by running water in its various forms, 

 and by the winds, avalanches, and the atmosphere in degrading aud fashioning the surface subse- 

 quent to the melting of the ice is as yet scarcely appreciable in general views, for the time it has 

 been exposed to the action of these forces is comparatively short, while the scored aud polished 

 remnants of the glacial surface that have survived their wasting action, and which are still only a 

 few inches above the level of the general surface, show how little post-Glacial degradation has 

 been accomplished. 



On the other hand, the quantity of material quarried and carried away by the force of ice in 

 the process of bringing the region iutoits present condition cau hardly be overestimated; for, with 

 the exception of the recent volcanic cones, almost every noticeable feature, great and small, has 

 evidently been ground down into the form of greatest strength in relation to the stress of over- 

 sweeping floods of ice. And that these present features are not the pre-Glacial features merely 

 smoothed aud polished and otherwise superficially altered, but an entirely new set sculptured 

 from a surface comparatively featureless, is manifested by the relationship existing between the 

 spaces that separate them aud the glacier fountains. The greater the valley or hollow of any sort, 

 the greater the snow-collecting basin above it whence flowed the ice that created it, not a fiord or 

 valley being found that does not conduct to fountains of vanished or residual glaciers correspond- 

 ing with it in size and position as cause and effect. 



And, furthermore, that the courses of the present valleys were not determined by the streams 

 of water now occupying them, nor by pre-Glacial streams, but by the glaciers of the last or of some 

 former Glacial period, is shown by the fact that the directions of the trends of all these valleys, 

 however variable, are resultants of the forces of the main trunk glaciers that filled them and their 

 inflowing tributary glaciers, the wriggling fortuitous trends of valleys formed by the action of 

 water being essentially different from those formed by ice ; aud therefore not liable to be confounded. 

 Neither can we suppose pre-existing fissures or local subsidences to have exercised any primary 

 determining influence, there being no conceivable coincidence between the trends of fissures aud 

 subsidences and the specific trends of ice-created valleys and basins in general, nor betweeu the 

 position aud direction of extension of these hypothetical fissures aud subsidences and foldings and 

 the positions of ice-fountains. 



It appears, therefore, in summing up the results of our observations here that a few active 

 glaciers still exist on the highest mountain of Ounalaska; that the ancient glaciers, in their 

 sheeted and distinct conditions, embraced all the Aleutian region , and sculptured its pre-Glacial 



