332 Correspondence— W. H. 8. Monch. 



■would ground, but it is not true of the shallower parts near the 

 shore on which the coast-ice acts and on to which floe-ice and pack- 

 ice is often driven with immense force, — agencies which seem to 

 be quite as capable of carrying with them the masses of partially 

 frozen materials and of pushing them over a floor of solid frozen 

 sand as Land- or Glacier-ice could be. 



Mr. Marr refers to a recently-described case where a glacier 

 traversing a narrow valley seems to have overfolded certain deposits 

 of stratified sand and clay ; thus comparing what may happen in 

 a narrow valley with the phenomena of a district of which he him- 

 self says "not only do the contortions occur in the drifts which 

 occupy the valley bottoms, but they are also found in the accumu- 

 lations which lie on the summits of ridges." Are we to suppose 

 that so able a geologist as Mr. Marr thinks an ice-sheet over-riding 

 a ridge will act in the same way as a glacier pushing itself through 

 a narrow valley ? 



The sections round Sudbury are exceedingly interesting, and Mr. 

 Marr deserves our thanks for calling further attention to them and 

 for recording new aspects of the changing pit-faces, but in his charge 

 to the jury he has not put all the possible alternatives, and conse- 

 quently his summing-up is biassed in favour of one explanation. 



June 6, 1887. A. J. Jukes-Bkowne. 



THE CAUSES OF GLACIATION. 



Sir, — I ask leave for a few remarks on the question of the causes 

 of glaciation, as there are some points connected with it on which I 

 think sufficient stress has not hitherto been laid. 



The total amount of direct solar heat received at any place is 

 admittedly nearly constant whatever be the eccentricity of the earth's 

 orbit. The amount indirectly received through the medium of air- 

 currents, clouds, and ocean -currents may vary ; but if the variations 

 of this indirect heat are ascribed to the raising or lowering of the 

 temperature, the causes of this raising or lowering must be sought 

 for in the distribution of the direct heat. We come, therefore, to the 

 question. What distribution of direct heat over the various seasons 

 (the total amount being unaltered) is most favourable to glaciation ? 



In the first place, then, it seems clear that the Glacial period could 

 not have been produced by the freezing of water in situ. A snow- 

 cap or ice- cap reaching an elevation of hundreds or thousands of 

 feet over the sea-level could only have resulted from falls of snow. 

 The former question is therefore resolved into the following, What 

 distribution of direct heat is best calculated to increase the annual 

 snow-fall ? 



In answering this question, two principles must be borne in mind. 

 First, that snow will not fall, or at least will not lie, if the tempera- 

 ture is much above freezing-point. In such cases either rain would 

 take the place of snow, or else the snow would melt at once. Second, 

 that very little snow falls when the temperature is very low. Great 

 cold preserves the snow that has fallen, but it seems necessary for a 



