538 Correspondence — Mr. David Forbes. 



them doubly valuable), and, consequently, I had great pleasure, when 

 perusing his letter, to find that he felt inclined to differ from me 

 only in a very few, and I might add, not very important, points. 



Having devoted some time to experimental researches upon vol- 

 canic action, and having had more than ordinarily good opportunities 

 of studying in the field the active as well as passive volcanos of 

 various parts of Europe, Africa, America, and Australasia, I attempted, 

 in the lecture alluded to, a resume of the conclusions I had arrived 

 at independently, but which in many points fully confirm some long 

 previously announced by Mr. Scrope himself; and had it not been 

 for the necessity I was under, of condensing so large a subject as 

 volcanos into the short space of an hour's lecture to a non-scientific 

 audience, and, consequently, compelled to omit all explanatory de- 

 tails, I believe that Mr. Scrope himself would not have felt inclined 

 to differ, even as to the few points to which he refers in his letter, 

 and to which I will now briefly allude. 



1. "With regard to the formation of volcanic sands, ashes, and dust, 

 I fully admit, with Mr. Scrope, that the greater part is probably due 

 to the attrition of the particles mter se during their repeated ascents 

 from, and descents into, the crater ; nevertheless, microscopical 

 examination shows that, in a very large qiiantity, the particles are 

 more or less fused or rounded externally, as well as fuU of air, gas, 

 or steam pores or bubbles, which would indicate that they have 

 been comminuted and, as it were, blown to pieces whilst in the 

 viscid, if not molten, state. 



2. The opinion that molten lava, when suddenly brought into 

 contact with water, is broken up instantaneously into coarser or 

 finer particles, even to mud, is founded, not only upon observation, 

 but on experiment also ; for when it is found that molten lava, 

 furnace slags, glass, and other silicates, do become so broken up 

 when poured out suddenly into cold water, I think it is but reason- 

 able and fair to infer that they would also so behave in nature, and, 

 therefore, demur to the assertion, "It is very difficult, not to say 

 impossible, to do more than guess at the effect produced on a body 

 of lava expelled from a volcanic vent beneath water." The tri- 

 turating action of the sea, which would naturally be enormously 

 increased in such convulsions, is not only admitted but assumed by 

 me as part of the process ; the whole drift of the paragraph in 

 question being to explain how great sedimentary deposits, composed 

 of volcanic matter, may be formed, often similar in appearance, but 

 infinitely more rapidly than those beds slowly built up from the 

 debris arising from the wearing away of terrestrial rocks, brought 

 down by the action of rain and rivers. 



3. As before alluded to, this is but an instance in which my con- 

 clusions are identical with those previously arrived at by Mr. Scrope, 

 and I feel sure that all unbiassed geologists must ultimately admit 

 that the internal agencies have played the most prominent part in 

 determining the external features of the earth. 



4. Lastly, Mr. Scrope objects to the use of the words Cataclysm 

 and Cataclysmic in contradistinction to Uniformity and Uniformitarian. 



