1871.] GEIKIE TEBTIARY TOLCANIC EOCKS. 293 



transmitted light, it is found to consist of a base of plagioclase 

 felspar, in minute, somewhat decomposed crystals, with abundant 

 black grains of titaniferous iron, and a brown, much decayed mineral, 

 which may be augite. The higher part of the bed, at Dunan Tha- 

 lasgair is darker in colour, and, when examined microscopically, 

 has much the character of an anamesite. Indeed the whole rock 

 might be regarded as a highly felspathic basalt-rock in which the 

 ferruginous silicates are poorly developed. 



This rock is, as a whole, strongly amygdaloidal, the cavities in the 

 upper part of the bed being sometimes so flattened and elongated as 

 to impart a kind of fissile texture to the mass. This is more par- 

 ticularly to be noted at the precipice of Dunan Thalasgair. Through- 

 out a considerable part of the bed, the calc-spar and zeolites of the 

 kernels have disappeared, and the rock has resumed its original 

 vesicular aspect. 



y. Tuffs, Breccias, Sfc, 



One feature which distinguishes the Tertiary volcanic series of 

 Britain from those of earlier geological periods is the comparative 

 paucity and thinness of the intercalated beds of fragmentary ma- 

 terials. Among the contemporaneous igneous masses of the Silurian, 

 Old Red Sandstone, Carboniferous, and Permian periods we find 

 within our own borders enormous beds of tuff and volcanic breccia or 

 conglomerate ; but among the great basalt-plateaux of our north- 

 western tracts such intercalations are represented by mere thin in- 

 frequent layers. This appears to be the case at least from the south 

 of Antrim to the north of Skye — the most important tuffs in that 

 extended area, so far as I am aware, being those of the cliffs at the 

 Giant's Causeway. In Eigg this comparative insignificance of the 

 fragmental as contrasted with the crystalline or lava-form rocks is 

 characteristically maintained. Throughout the greater part of the 

 cliff-sections one bed of dolerite or basalt foUows another without 

 the intervention of any dividing layer of tuff or other deposit. Here 

 and there, indeed, between the beds, we not unfrequently meet with 

 a thin irregular seam of red earth, which, when fine, might be 

 called bole. In the cliff below Dunan Thalasgair, for example, 

 several of the dolerite -beds are not only covered by this substance, 

 but seem to pass into it. This may be observed also throughout the 

 Inner Hebrides, and conspicuously along many parts of the Antrim 

 coast-line. I have recently observed a precisely similar red parting 

 between several of the lava-streams which have been laid open by 

 the sea, and by artificial excavations, between Naples and Pompeii ; 

 and I may add that it is likewise to be observed between the sheets 

 of melaphyre interbedded with the lower carboniferous rocks of 

 Kinghorn, in Pife. In aU these cases I regard this red layer as 

 marking a surface of the igneous rock, decomposed into clay or soil 

 by exposure, and subsequently heated and altered by the overflow 

 upon it of the next sheet of molten material. 



At the north end of Eigg, along the cliffs of Beinn Bhuidh, a bed 

 of coarse doleritic or basaltic breccia is interstratified with the other 



