254 PROCEEDINGS OF THE ACADEMY OP [1883. 



force of the rolling stones against the roofs of the glaciers, must 

 haA'e some influence on its descent, as also would the weight of 

 water under the snow forming the cap, pressing against it at the 

 highest point of the glacial departure. The roof of the glacier 

 above the torrent would possibly get worn away somewhat by the 

 friction of the torrent ; but as ice is now known to be ductile, it 

 w^ould bend down towards the water when any great hollowing 

 out occurred, and get aid in its downward flow. We may further 

 imagine tiat under such an explanation as this, the edges pf the 

 glacier would have much more of excoriating power, than when 

 the whole mass is spread equally over a wide rocky bed. 



In regard to the existence of the glaciers, Mr. Meehan observed 

 that in many instances there were evidences of rapid retreat. 

 Davidson's glacier, at the head of Pyramid Harbor, near the 

 mouth of the Chilkat River, in about lat. 59^, had fallen back 

 several miles from the water in tlie bay. Having but little more 

 than half a day on shore at this point, an effort to reach the mouth 

 of the glacier failed through taking a "short cut" through a 

 forest of alder and spruce, the undergrowth of the spiny Pana 

 hon-ida being almost impassable. But field-glass observation from 

 the vessel, together with the examination of the track of the 

 retreating ice, showed successive terraces of moraine material, 

 with succeeding generations of trees on them in the supposed 

 distance of three miles from the sea to the glacier's mouth. Near 

 the glacier the trees appeared to be about twenty or twenty-five 

 years old ; nearer the sea, from seventy-five to one hundred. But 

 here, as in the Muir glacier, there were evidences of frequent 

 advances and of retrocession in the glacial material. Trees which 

 from their size may have been from thirty' to fifty years of age, 

 would have a deposit of twenty or thirty feet of material placed 

 around them, half burying them, and then again have it all cleared 

 away, leaving the dead trunks to tell the story. 



The volume of water now flowing in the line vacated by the 

 glacier, is not near equal to the work which has been done in 

 former times ; and the less quantit}' with the retreat of the glacier 

 itself, while other glaciers not fifty miles away still continue their 

 connection with the water, shows that local causes may be at work 

 which maj^ either retard or accelerate a glacier's progress. As 

 alread}' noted, the warmth of the atmosphere near a glacier's mouth 

 will, in a great measure, depend on the volume of cold water pro- 

 jected into the ocean — the greater the volume, the more influence 

 on the warm current which must be drawn in to take its place ; 

 and this is as true of the atmosphere as of the water. The heavy 

 cold body pushes the higher w^armed air upvvards, which has to 

 take the place of the air which rolls forward towards the lighiened 

 spot. Hence the greater the volume of cold air departing, the 

 larger and s-tronger the current of lighter and warmer air which 

 returns to the source of motion, so the temperature is not low in 

 the vicinity of the glaciers. On the iceberg before described, the 



