The Theory of Glacier motion. 95 



melting it cannot move. True, when liquid it is reduced 

 about one-tenth in size, and in consequence its centre may 

 move a small fraction of its diameter towards A ; but on 

 freezing again it must resume its original position. A or C 

 now gets the heat, melts, oscillates, and freezes in its old 

 position ; and so on. From such heat there is plainly no 

 molecular motion." Even if we were to modify Mr. Croll's 

 theory so as to make the freezing of one particle synchronous 

 with the melting of its neighbour, we should secure only the 

 smallest imaginable molecular motion. In reference to the 

 denuding power of glaciers, which Mr. Croll explains as 

 due directly to the stones and other hard matters embedded 

 in them and propelled along by the molecular movement 

 he appeals to, Mr. Burns replies forcibly there could not 

 be a weaker denuding agency than a great glacier which as 

 a solid mass is stationary, but on whose interior and on 

 whose surface liquid molecules are here and there moving 

 through infinitesimal distances .... and will a stone that is 

 held in the crystalline grasp of millions of ice molecules be 

 forced along by a few dozen water molecules trickling 

 through a fraction of their diameter along its surface. A 

 stone thus * forced along,' may be supposed to scoop out 

 valleys if the exigencies of geologists demand it, but the 

 force that moves the stone would not serve to tickle the 

 sole of a mite .... The motion of the molecules within a 

 glacier can no more cause a thrust than the rise of the sap 

 within a tree in spring can pull it up by the roots." 



Dr. Croll did not answer this attack, but in a subsequent 

 paper he professes to have abandoned the views he originally 

 propounded on the subject, and to have modified his position 

 considerably. In this paper he says : " Ice is not absolutely 

 solid throughout. It is composed of crystalline particles, 

 which, though in contact with one another, are however not 

 packed together so as to occupy the least possible space, 

 and even though they were, the particles would not fit so 



