442 Correspondence — Mr. G. Poulett Scrope. 



up, to reach the open air. In either case the disturbance of the sea- 

 water — whether at great depths through the repeated outburst and 

 immediate condensation of huge bubbles of steam, or by the additional 

 action of the waves when the mouth of the volcano had risen high 

 enough to be affected by them — would amply account for the tritur- 

 ation of the fragmentary lava into the materials of ash or mud, as 

 well as its spreading over large contiguous areas. 



Thus, whether in the case of sub-aerial or subaqueous volcanic 

 eruptions, I see no reason for attributing the fine division of the 

 mineral matter which composes the fragmentary lava-rocks, com- 

 monly called volcanic ash, tuff, peperino, or puzzolana, to any 

 sudden or instantaneous process, but to simple mechanical tritura- 

 tion, or the rubbing against each other of a crowd of coarse particles, 

 forcibly set in motion, whether in the air or water. 



Since, as I before observed, such fragmentary matter constitutes at 

 least one moiety of all volcanic formations, of whatever age, the 

 question of its mode of production is not unimportant. 



3. With respect to the controversy as to the causes which have 

 shaped out the terrestrial surfaces of the globe, I need only refer to 

 the short paper which appeared in your number for June, 1866 

 (Geol. Mag., Vol. III., p. 241), to prove my entire accordance with 

 the argument of Mr. Forbes (p. 327), that "the most prominent part 

 in the determination of the external features of the earth " must be 

 assigned " to its internal agencies," to which alone is owing the 

 occasional elevation of great masses of land up to and often far above 

 the sea-level, and their consequent exposure to the sculpturing and 

 denuding influences as well of ocean waves and currents, as of the 

 meteoric sub-aerial forces — rain, rivers, frost, glaciers, etc.'' 



4. The only other remark I wish to make on Mr. Forbes's lecture 

 has reference to the mode in which he, as others have done before 

 him, employs the word "Cataclysm," or "Cataclysmic," as involving 

 an idea opposed to, or inconsistent with, that of " Uniformity " 

 (p. 327, line 21). 



I understand by a " Cataclysm " exactly what in my work on 

 Volcanos I have preferred to call a " Paroxysm," i.e., a more than 

 ordinary violent event in the physical history of the globe, as for 

 example, an extraordinary earthquake, volcanic eruption, or flood, 

 such as occurs perhaps but once in a century or even in a thousand 

 years, but which is still perfectly consistent with the general uni- 

 formity of the terrestrial and cosmical agencies. The theory which 



1 I observe that Colonel Greenwood, in your last number (p. 395), ridicules the 

 phrase "meteoric," as applied to the sub-aerial denuding agencies, instead of '^rain 

 and rivers." But can he not see that his favourite formula leaves out the influences 

 of alternate frost and thaw, of snow and ice, of electricity, chemical decomposition, 

 etc., all more or less effective in wearing away the exposed surfaces of land ? The 

 Colonel seems, too, to claim for himself the origination of what he calls "the rain 

 and river theory of erosion." Now, without denying the "originality" of much 

 in Colonel Greenwood's volume, yet its first edition was printed only in 1857, and 

 geologists are well aware that through half a century before that date the doctrine of 

 the enormous influence of atmospheric agencies (rain and rivers inclusive) in mould- 

 ing the surface of the earth had been earnestly advanced by Huttou, Playfair, Lyell, 

 myself, and others. (See Quarterly Keview for June, 1827, p. 477.) 



