328 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [March 22, 



occurrence of gravel-beds under lava-currents in Auvergne and else- 

 where. Sir Charles supposes that on the flanks of Madeira, as on 

 the slopes of Etna, or Mount Loa in the Sandwich Islands, there 

 were no rivers. At Porto da Cruz, however, there are well-worn 

 pebbles in some comparatively modern tuffs, covered by trachyte ; 

 and there are also some partially rounded stones in alluvial tuffs, of 

 relatively modern date, at Camara de Lobos near Funchal. Yet 

 neither here nor elsewhere in the island do the lavas overlying allu- 

 vium follow the windings of existing valleys, as in Auvergne. The 

 Curral and other deep valleys are ascribed to aqueous erosion, chiefly 

 fluviatile, assisted by subterranean movements. 



In one of the letters the sea-cliff section of Cape Giram, 1600 feet 

 high, was described. Several hundred dikes are there seen, intersect- 

 ing scorise, tuff, and lava. In another letter the volcanic cone of 

 Piedade, near Cani^al, is spoken of. Its structure has been exposed 

 by encroachments of the sea on the coast near the eastern extremity 

 of the island. Near Cani9al also, and in Porto Santo, thirty-five miles 

 N.E. of Madeira, ancient sand-dunes occur, containing fossil land- 

 shells, chiefly of living, but in part of extinct species. 



The rocks of Porto Santo are both basaltic and trachytic. Those 

 of the little island of Baxo, south of Porto Santo, are of submarine 

 volcanic origin, covered by supra-marine lavas. In the tuffs of the 

 older series, corals and shells abound, and water-worn pebbles, such 

 as may have been formed on a sea-beach ; and these are cemented 

 together by carbonate of lime in the same conglomerate with the 

 organic remains. 



2. On Yedmandale, as illustrating the Excavation of some 

 Valleys in the Eastern Part of Yorkshire. By H. C. 

 SoRBY, Esq., F.G.S. 



The Tabular Hills of the Eastern Moorlands of Yorkshire in their 

 eastern part, that now under consideration, are very flat-topped ; 

 gradually rising from the vale of Pickering, on the south, where the 

 elevation above the sea is about 100 feet, and attaining a height of 

 about 600 feet at the escarpment six miles north of this, overlooking 

 Harewood dale. The surface rock of the southern part is coralline 

 oolite, which, however, has been removed from the northern part, 

 where the calcareous grit is laid bare. Numerous ravine-like valleys, 

 whose sides are inclined at an angle of about 30°, have been excavated 

 in them, chiefly trending N.N.W. ; but some are at right angles to that 

 direction, being respectively nearly in the lines of the dip and strike 

 of the beds, which, on the whole, lie nearly parallel to the surface. 

 Many of these have been much modified by the streams now running 

 in them, and present such appearances as might lead one at first to 

 conclude that they had been altogether excavated by their action ; 

 and, indeed, there is nothing in their structure to prove decidedly 

 that such was or was not the case. The peculiarity of Yedmandale, 

 however, is, that there appears never to have been any brook or 

 stream of modern drainage in its upper eastern branch, and that it is 



