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



Except in the elongation of the vesicles in one general direction, 

 the amygdaloidal basalts seldom display any distinct trace of flow- 

 structure. Occasionally, however, a striking exhibition of this 

 structure may be seen among them. Thus at the top of the Dun 

 Can, the highest summit of the island of Raasay, a small outlier of 

 the plateau-lavas is capped with a black olivine-basalt, having a 

 strongly-pronounced vesicular structure, wherein the cavities are 

 filled with zeolites. The weathered faces of this rock show rudely 

 parallel, puckered, and broken lines that mark the layers of devitri- 

 fication in the original flowing lava. 



(3) Prismatic or columnar basalts, which, as at the Giant's 

 Causeway and Staffa, have long attracted notice as one of the most 

 notable topographical elements in the structure of the plateaux. 

 Though generally rather compact, becoming indeed dense, almost 

 vitreous rocks in some sheets, they are often more or less cellular 

 throughout, and highly slaggy along their upper and under surfaces. 

 In some cases, as in that of a prismatic sheet which overlies the 

 rough scoriaceous lava of Camas Tharbernish just referred to, the 

 rows of vesicles are disposed in lines parallel to the under surface 

 of the sheet (fig. 1). 



Fig. 1. — Section of scoriaceous and prismatic basalts. Camas 

 Tharbernish, northern shore of Canna Island. 



As I have already remarked with regard to the massive, rudely- 

 jointed sheets, many of the most perfectly columnar rocks of the 

 plateaux are not superficial lavas, but intrusive sills, bosses or 

 dykes. Conspicuous examples of the sills are displayed on the coast 

 of Trotternish in Skye, of the bosses and dykes at the eastern end 

 of Canna. To these further reference will be made in the sequel. 

 It is not always possible to be certain that columnar sheets which 

 are regularly intercalated among the undoubted lavas of the volcanic 

 series may not be really intrusive. In some instances, indeed, we 

 can demonstrate that they are so, when, after continuing perfectly 

 parallel with the lavas above and below them, they eventually break 

 across them. One of the most remarkable examples of this feature 

 is supplied by the great sill of the south-west of Stromo in the 

 Faroe Islands, of which I shall give some account in a subsequent 

 section of this paper (figs. 9, 24, & 25, pp. 345, 380, 381). 



(4) Eanded or stratiform lavas, consisting of successive parallel 

 layers or bands which weather into projecting ribs and flutings. 

 The deceptive resemblance to sedimentary rocks thus produced 



