Reports and Proceedings — Geological Society of London. 381 



We cannot think lie has acted wisely in substituting De Lapparent's 

 ' Gotlandian ' for Upper Silurian. The student should be taught to 

 use the generally accepted terms. Silurian drops out entirely from 

 Professor Cole's Table of Formations (p. 293), 



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Geologioal Society of London. 



June 11th, 1902.— Professor Charles Lapworth, LL.D., F.R.S., 

 President, in the Chair. 



Professor Bonney exhibited a mounted specimen of the volcanic 

 dust which fell on the deck of the steamer Boddam during the great 

 eruption of Mont Pelee on May 8th, for which, as well as for another 

 from the Soufriere of St. Vincent, that had fallen in Barbados, he 

 was indebted to Sir William Crookes, F.E.S. The dust from Mont 

 Pelee consists of fragments of minerals and rock (the former, 

 perhaps, slightly in excess of the latter), very commonly about 

 •007 to -008 inch in diameter, but ranging from about -005 to 

 •01 inch. A very little fine dust had been removed by levigation 

 before mounting the specimen. The minerals are : — (1) Chips of 

 felspar sometimes bounded by cleavage-edges, occasionally showing 

 oscillatory twinning or zonal structure. The refractive index and 

 extinction-angles suggest that the majority are labradorite. Some 

 contain minute acicular microliths or small brownish enclosures 

 (? vitreous), which now and then are regular in form and 

 arrangement, like negative crystals, and not seldom contain little 

 bubbles. (2) Pyroxene, occasionally with cleavage-edges, or even 

 idiomorphic, generally of a light bottle-green tint. There are 

 certainly two species : one showing a distinct pleochroism from 

 green to brown with straight extinction — a variety of hypersthene ; 

 the other barely pleochroic, with an extinction that proves it to 

 be augite. He could not identify with certainty magnetite or 

 any other mineral. The rock-fragments are chips of a brownish, 

 often dirty-looking glass, with small cavities, sometimes showing 

 microliths or adhering to minerals ; much of it opaque, or nearly so, 

 with transmitted light, and a brownish-grey by reflected light, once 

 or twice reddish. As Dr. Flett gave an excellent description of the 

 Barbados dust from the Soufriere at the previous meeting, the present 

 speaker thought that he need say no more than that in the specimen 

 now exhibited the fragments seem a shade smaller, and minerals 

 are slightly more abundant, especially pyroxene, than in the Mont 

 Pelee dust. 



Notwithstanding the risk of generalizing from a single slide. 

 Professor Bonney inferred that the ejecta of the two volcanoes 

 are generally similar. Both, compared with specimens in his 

 cabinet from Cotopaxi, are more uniform in size. The travelled 

 dust from the Soufriere is a little smaller than that from the actual 

 summit of the Andean volcano, but coarser than similar material 

 from Chillo (over 20 miles), Quito (35 miles), Ambato (45 miles), 

 Eiobamba (65 miles), and the summit of Chimborazo, about the same. 



