Vol. 52.] BASALT-PLATEAUX OF NORTH- WESTERN EUROPE. 333 



surfaces of such sheets among the Tertiary basalt -plateaux must 

 have resembled the so-called * Aa ' of the Sandwich Islands. A 

 striking example of the structure may be noticed at Camas Thar- 

 bernish, on the northern coast of the island of Canna. There the 

 hummocks on the upper surface of a slaggy basalt measure about 15 

 feet in breadth, and rise about 3 feet above the hollows between 

 them like a succession of waves (see fig. 1, p. 334). The steam-holes 

 are disposed in a general direction parallel to the strike of the 

 hummocks. 



Great variety obtains in the size and shape of the vesicles. Huge 

 cavities a foot or more in diameter may occasionally be found, and 

 from such an extreme every gradation may be traced down to 

 minute pore-like vacuoles that can hardly be made out even with a 

 strong lens. In regard to the deformation of the vesicles, it is a 

 familiar general rule that they have been drawn out in the direction 

 of the flow of the original lava. Occasionally this elongation has 

 advanced so far that the cavities have become straight, narrow 

 pipes, several inches long, and only an eighth of an inch or so in 

 diameter. A number of such pipes, arranged parallel to each other, 

 resembles a row of worm-burrows. Eemarkable illustrations of 

 such extreme mechanical deformation by the movement of a still 

 molten rock may be collected in Mull and Skye. 1 



It is a common belief that the filling-in of the steam-cavities 

 has taken place long subsequent to the volcanic period by the slow 

 percolation of meteoric water through the rocks. I believe, how- 

 ever, that at least in some cases, if not in all, the conversion of the 

 vesicular lavas into amygdaloids was effected during the volcanic 

 period. Thus it can be shown that the basalts which have been 

 disrupted by the gabbros were already amygdaloidal before these 

 basic intrusions disturbed them, for the kernels of calcite, zeolite, 

 etc., have shared in the general metamorphism induced in the 

 enclosing rock. Again, the blocks of amygdaloid contained in the 

 agglomerates of the volcanic series are in every respect like the 

 amygdaloidal lavas of the plateaux. It would thus seem that 

 the infilling of the cavities with mineral secretions was not 

 merely a long secular process of infiltration from the cool atmo- 

 sphere, but was more rapidly completed by the operation of warmer 

 water, either supplied from volcanic sources or heated by the still 

 high temperature of the cellular lavas into which it descended from 

 the surface. 2 



1 Some examples have been deposited by me in the Museum of Practical 

 Geology, Jermyn Street, in the case illustrating rock-structures. The elonga- 

 tion of the vesicles into annelid-like tubes may be observed among the stones 

 in the volcanic agglomerates. 



2 Messrs. Harker and Marr have demonstrated that the Lower Silurian 

 vesicular lavas of the Lake District had already become amygdaloids before 

 the uprise of the Shap Granite, Quart. Journ. Geol. Soc. vol. xlix. 1893. 

 J. D. Dana, originally an advocate of infiltration from above, subsequently 

 supported the view here adopted, that the kernels of amygdaloids were filled in 

 by the action of moisture within the rocks during the time of cooling (Am. 

 Journ. Sci. ser. 3, vol. xx. 1880, p. 331). 



