548 J. It. Dakyns 8^ E. Greenly — Felsitic Slates of Snoicdon. 



Now, those who have read the fascinating report of Drs. Anderson 

 and Flett on the West Indian Eruptions of 1902, will remember that 

 absence of stratification is stated to be a marked characteristic of the 

 deposits of St. Vincent, and is ascribed by them to the sudden and 

 torrential or avalanche-like manner in which they were poured out.' 



It is true that the phenomena which have given such a dramatic 

 interest to these eruptions — the furious rush, the deadly blast, and 

 the great black cloud — are essentially subaerial, and that the volcanic 

 series of Snowdon is generally regarded as submarine. But as the 

 conditions for the type of eruption, called by the authors of the 

 report the Pelean, are determined in the throat of the volcano before 

 emergence, there is nothing to prevent such eruptions taking place 

 from submarine vents. 



The felsitic slates, therefore, present in a striking manner one of 

 the leading features of a Pelean deposit. 



But there is another characteristic of a Pelean dust, equally 

 important, and that is the high proportion of crystalline to glassy 

 grains. 



An ancient Pelean deposit ought, therefore, to be a largely 

 crystalline deposit, and if not too much altered ought to retain 

 some traces of this. The rocks under discussion, as we have 

 shown, are for the most part much too highly altered for any but 

 their larger original grains to be recognisable, but these grains are 

 crystalline, and so far agree. 



The fine matrix, in -the highly altered rocks, might have been 

 originally either crystalline or glassy, and proves nothing either 

 way. But the fossiliferous specimen, we have seen, is much less 

 altered ; to it, therefore, we may appeal. Parts of it now yield 

 aggregate polarisation, and these may, and very likely do, represent 

 particles originally glassy, now devitrified, and there is even a little 

 isotropic matter still surviving. But the greater part consists of 

 broken felspars set in a dusty matrix in which smaller and smaller 

 clastic, crystalline, particles can be detected, until we reach the limit 

 of vision. 



The dust is a crystalline dust, and, in addition to this, the groups 

 of biotite crystals, cemented by what appears to have been glass, 

 recall the lapilli with microliths and the crystals with glass adhering 

 to them of the typical Pelean products of 1902. 



Such evidence as we have, therefox'e, goes to confirm the view, 

 suggested in the first instance by their behaviour in the field, that in 

 the felsitic slates of Snowdon we have a Pelean deposit of the 

 Ordovician period. 



Concluding Considerations. 



One or two points call for remark. The particles, except the 

 larger felspars, are decidedly smaller than those of the West Indian 

 dust-clouds, even than what was collected at Barbadoes, some ninety 



1 Phil. Trans. E.S., 1903 : Eeport on the Eruptions of the Soufriere, etc., 

 pp. 448, 470. 



