D. Burns — On the Mechanics of Glaciers. 297 



Norway, but no example of so extensive a spreading of volcanic 

 ashes with the wind, as from Iceland to the east coast of Sweden, is 

 previously known. On Iceland the ashes fell in such quantity that 

 at some places they covered the ground to a depth of six inches and 

 destroyed the pastures. The cloud of ashes was for several hours so 

 close that the sunlight could not penetrate it, and lights required to 

 be kindled in the middle of the day. The ashes must also have 

 fallen in considerable quantity in the sea between Iceland and 

 Norway, and on its bottom there are doubtless found places where 

 the remains of such falls collect during centuries without any con- 

 siderable mixture of foreign matter. Here must be formed thick 

 beds of volcanic ashes, which in the course of geological ages 

 gradually harden together, and are metamorphosed to rocks of 

 nearly the same composition, and therefore also strongly resembling 

 those which in molten form burst forth from the interior of the 

 earth, and we have here doubtless the key to the extension over 

 boundless regions of the earth of stratified so-called volcanic rocks, 

 a circumstance to which I have already long ago drawn attention 

 with reference to the occuiTence of plutonic rocks regularly stratified 

 in the polar regions. 



III. — On the Mechanics of Glaciers. 



By David Burns, 

 Of H.M. Geological Survey. 



No. 1. 



IT has long appeared to me that much remains to be said on this 

 subject. When Mr. Croll's papers on the Physical Cause of the 

 Motion of Glaciers appeared in the Philosoj)liical Magazine for 

 March, 1869, and September, 1870, I felt that, though his theory 

 was certainly ingenious, and his statement of it clear and masterly, 

 these papers were far from affording a satisfactory solution of the 

 problem of which they treated. I waited, fully expecting that Mr. 

 Croll would supplement and modify his theory, but he has not done 

 so to my knowledge. Geologists in general seem to be as satisfied 

 with it as he is himself, and on all hands they appeal to it as sup- 

 porting them, whenever they treat of glacial phenomena, even when, 

 if true, it shows their theories to be vain. A rather conspicuous 

 instance of this occurs in the Geol. Mag. for October of last year, in 

 an interesting paper by Mr. Goodchild on Coums. In that paper 

 Mr. Goodchild commends "a proper appreciation of Mr. Croll's 

 theory" ; but I am persuaded that a proper understanding of it would 

 have caused him either to renounce that theory or his own on the 

 origin of Coums. I have thought it time, therefore, to place on 

 these pages my criticisms and difficulties for the consideration of 

 geologists, even if, as is unfortunately too likely, I have little new 

 to advance. 



Let us now turn to Mr. Croll's first paper on the Motion of 

 Glaciers. And first let us trace the molecular melting, which is 

 indeed the whole theory. He supposes that a molecule A at the 



