306 Prof. Spencer — The Pacific Ocean and Gulf of Mexico. 



rhyolite, with banded structure, comparable in type to the " mill- 

 stone-porphyry " of Hlinik in Hungary, but of somewhat coarser 

 character. At the west end, on the airy summit of the moor, 

 rhyolite of the Tardree type crops out ; and in between, near 

 a hollow of the road, its glassy representatives abound. The banded 

 and fluidal pitcbstones of Skleno are here beautifully repeated; and 

 dull-brown spherulitic aggregations also occur, their cracks and. 

 hollows being filled, as in Hungary, with chalcedony and common 

 opal.^ No continuous mass of glass is visible, and in one pit the 

 obsidian boulders resemble blocks that became included in a more 

 crystalline flow, the latter having now decomposed to form a sand, 

 A resisting layer of pale pink rhyolite occurs a few yards farther 

 north, and no doubt extends down into the hill ; and above it, and 

 in contact with it, is an agglomerate of glassy and dark hemi- 

 crystalline fragments of rhyolite, embedded in a firm yellowish 

 ground. This can hardly be anything but a true tuff; and many of 

 the sands containing obsidian boulders in this district may have been 

 formed by the breaking up of similar layers. The exposure is a 

 small one, and this interesting relic may disappear under a few 

 weeks' quarrying. It became revealed, however, between two 

 successive visits made in 1894 and 1895 ; and at any time 

 further evidence of explosive action may be met with in the surface- 

 diggings of Sandy Braes. 



Nothing short of deeper trenches or more serious quarrying can 

 show us the real wealth of the material on the heights north of 

 Tardree Mountain ; but I trust T have said enough to sustain my 

 main contention. I regard the Tardree dome as the neck formed 

 by the upwelling of a viscid rhyolite, which occasionally flowed 

 out over the Chalk, and perhaps over some of the earlier basalts, 

 round about it. A considerable volcano was in time built up, its 

 flanks formed of true lava-flows and even of tuffs; glassy crusts were 

 developed freely by the rapid cooling of these flows, and various 

 types of obsidian were spread out over some four square miles. 

 The lava-flows have been much reduced by denudation, and are 

 now obviously crumbling away on the high moor of Sandy Braes ; 

 but both here and under the later basalt of Carnearney we have 

 evidence of the former complexity and variety of the rhyolitic 

 volcano of Tardree. 



V. — Preliminary Notes on the late Connection and Separation 



OF THE Pacific Ocean and Gulf of Mexico. 



By Prof. J. W. Spencer, M.A., Ph.D., B.A.Sc, F.G.S. 



HAVING recently returned from another season's work in the 

 West Indies and Mexico, where I was collecting additional 

 data bearing upon the stupendous changes of level of land and sea 

 which have lately affected the American continent, I find the 

 review of the " Reconstruction of the Antillean Continent " by 

 Mr. Jukes-Browne in the Geological Magazine, April 1895, p. 173, 

 a few points of which may be further explained at the same time 

 1 Compare Berger, op. cit., p. 190. 



