J. JR. Bakyns S^ E. Greenly — FeMtic Slates of Snowdon. 549 



miles away from the volcano. Possibly an acid magma may dissipate 

 itself into finer debris than an intermediate one. 



Then another point — and we confess to feeling somewhat staggered 

 by this — is the enormous size of the mass. In the cliffs of Lliwedd 

 alone several hundred feet of the deposit can be seen, and this may 

 not be all, even if the rest of the horizon, estimated by Eamsay at 

 1,700 feet in all, be largely composed of lavas. 



Another circumstance shows a curious coincidence. Drs. Anderson 

 and Flett remark (Eeport, pp. 400, 505) that the Pelean type of 

 eruption appears to be generally followed, when once the great 

 black cloud has rolled away, by a ' rain of ashes ' of the ordinary 

 kind. Now in Snowdon itself it is certainly the case that the 

 felsitic slates are succeeded by the bedded ashes of the upper part 

 of the volcanic series. 



As to the fossils, one could not, indeed, expect to find them in the 

 heart of a thick deposit of Pelean dust. They were found in some 

 thin bands which alternate with the upper ashy series near its base. 



In one respect the hypothesis here put forward relieves the 

 geology of Snowdon from a difficulty that has been felt, namely, 

 the great extent of the felsitic lavas. For the horizon has been 

 traced over a tract of country some fifteen miles long by nearly ten 

 in width, and even then it ends off in escarpments. Certainly it 

 cannot be less than 150 square miles.^ It is difficult to believe that 

 acid lavas, with their well-known viscosity, could be continuous over 

 such a distance. Mr. Harker suggests ^ that under water they might 

 retain fluidity much longer than in air. But the fluidal structure, 

 so well marked and so common in them, shows that after all they 

 could not have been much less viscous than other acid lavas. If, 

 however, the * Lower Felstone ' horizon be composed to any great 

 extent of pyroclastic matter, the individual lavas may be com- 

 paratively limited in extent, and emitted from several vents, now 

 concealed beneath each other's productions. 



Finally, it may be remarked that ancient Pelean deposits will 

 easily escape detection unless they are thick, for a foot or two of 

 homogeneous unstratified tuff occurring in a volcanic series would 

 hardly attract attention. Yet the violent outbreaks from Pelee 

 itself of May 8th and July 9th, 1902,=^ even that which destroyed 

 St. Pierre, seem to have left only two or three feet of dust upon 

 the surface. 



However these things may be, and whether the view advocated 

 in this paper proves to be correct or not, it is certain that the work 

 of Drs. Anderson and Flett, and of the French and American 

 geologists who have described these phenomena, will give a fresh 

 stimulus to the study of volcanic action, and lend a new interest to 

 many of our ancient volcanic deposits. 



^ Mr. Harker's estimate is much greater. 



2 "Bala Volcanic Series," p. 121, quoting Scrope and others. 



2 The mass ejected from the Soufriere was much greater. 



