332 SIR A. GEIKIE ON THE TERTIARY [May 1 896,. 



As the literature of the subject was fully summarized in my 

 former memoir, it need not be further referred to here. Such more 

 recent papers as bear on any of the localities which I shall have to 

 describe, or on any of the questions I shall discuss, will be cited as 

 the occasion arises. 



I. The Plateau-Lavas. 



Every tourist who has sailed along the cliffs of Antrim, Mull, 

 Skye, or the Faroe Islands is familiar with the singular terraced 

 structure of the great volcanic escarpments which stretch as mural 

 precipices along these picturesque shores. Successive sheets of lava, 

 either horizontal or only gently inclined, rise above each other from 

 base to summit of these walls, as parallel bars of brown rock with 

 intervening strips of bright green grassy slope. 



The geologist who for the first time visits these coast-lines is 

 impressed by the persistence of the same lithological characters 

 giving rise to the same topographical features. He soon realizes 

 that the plateaux, so impressively truncated by the great escarp- 

 ments that spring from the edge of the sea, are built up essentially of 

 dark lavas, — basalts, dolerites, and andesites, — and that fragmental 

 volcanic accompaniments, though here and there well developed, 

 play on the whole a quite insignificant part in the structure and 

 composition of those thick piles of volcanic material. Closer exa- 

 mination in the field enables him to ascertain that, regarded as rock- 

 masses, the lavas include four distinct types : — 



(1) Thick, massive, prismatic or rudely -join ted sheets, rather 

 more coarsely crystalline and obviously more durable than the other 

 types — inasmuch as they project in tabular ledges and tend to retain 

 perpendicular faces, owing to the falling away of slices of the rock 

 along the lines of vertical joint. Many rocks of this type are 

 undoubtedly intrusive sheets, and as such will be further referred to in 

 a later part of this paper. But the type includes also true superficial 

 lavas which show the characteristic slaggy or vesicular bands at 

 their upper and lower surfaces. The mere presence of such bands 

 may not be enough, indeed, absolutely to establish that the rock 

 possessing them flowed at the surface as a lava, for they are occa- 

 sionally, though it must be confessed rarely, exhibited by true sills. 

 But the rough scoriaceous top of a lava- stream, the presence of 

 fragments of this surface in the overlying tuff, or wrapped round 

 by the next succeeding lava sufficiently attest the true superficial 

 outflow of the mass. 



(2) Slaggy or amygdaloidal lavas without any regular jointed 

 structure, but often with roughly scoriform upper and under layers, 

 and tending to decay into brown earthy debris. Some of the upper 



many points of geological interest met with in our cruises. The pleasant hos- 

 pitality of Mr. Thorn, the proprietor of Canna, enabled me to survey in great 

 detail on the 6-inch map of the Ordnance Survey the deeply interesting 

 geology of his island-home, and to revisit Hysgeir, while Miss Thorn took 

 photographs for me of some of the more striking geological features of Canna 

 and Sanday. 



