590 Mr. Mallet on the Briitleness and 



fall, — I am nevertheless impressed with the belief that the 

 main views I then enunciated as to glacier motions are true ; 

 they appear to be supported by Mr. Hopkins's researches, 

 and I am not aware that they conflict with any truth that Mr. 

 Forbes has advanced. 



To the latter philosopher will ever belong the merit and 

 the good fortune of having been enabled first to apply accu- 

 rate measures to glacier motion ; and by fixing our data and re- 

 moving a mass of rubbish, done more than any living man to 

 advance our knowledge of this subject. 



Dublin, April 9, 1845. Robert Mallet. 



Note 071 the preceding Paper. 



Since the above paper was written and read, it has been 

 urged as a forcible argument against the views I therein put 

 forward, as to the impossibility of plasticity existing in a cry- 

 stallized mass, that the mass of glacier ice may be viewed 

 as a sort of crystalline sponge, penetrated by water held be- 

 tween its interstices in all directions and at all depths, and 

 that, although a perfectly crystallized mass cannot be also 

 plastic, such a compound structure may admit of plasticity. 

 This I believe is the most that can be said against my views 

 above, and upon this I would remark, that it is mere matter 

 of assumption that the mass of glacier ice is throughout pene- 

 trated in any such way by watery spaces. Hand specimens 

 show nothing of the sort at whatever depth taken, nor have 

 the experiments made as to capillary fissures at all proved 

 either their universality as to depth or even surface, much less 

 their size being sufficient to permit plastic motion. Thawing 

 ice has never been found in other circumstances in any analo- 

 gous state; on the contrary, it is well known that when float- 

 ing icebergs descend into southern latitudes, and are in a 

 thawing condition, in place of being plastic they are through- 

 out their whole masses so pre-eminently brittle, that the firing 

 of a gun is often sufficient totally to dislocate and shatter their 

 scarcely coherent crystals. Nor does the unsound and slushy 

 surface ice found in our own or in Arctic climates, -when in a 

 thawing condition and greatly penetrated with water, present 

 any trace of plasticity. 



Again, it is confessed on all hands that the whole glacier 

 mass must be viewed as fidl of dislocations, and that the great 

 fragments are more or less insulated masses. These are often 

 100 or even 500 feet in depth. Is it then conceivable, that if 

 such masses were really plastic they should show their plasti- 

 city only in the direction of horizontal or inclined motion ? 



