Vol. 52.] BASALT-PLATEAUX OF NORTH- WESTERX EUROPE. 



361 



doubtful. It is sensibly isotropic, and of refractive power distinctly 

 lower than that of felspar. These characters would agree with 

 analcime, which is not unknown as a contact-mineral ; but it is 

 difficult to understand how analcime, even a lime-bearing variety 

 like that of Plas Newydd, 1 could be formed in abundance from 

 palagonitic material. An alternative supposition, perhaps more 

 probable, is that the clear substance is a glass, modified from its 

 former nature especially by the expulsion of the iron oxide into 

 the remaining matrix. A comparison is at once suggested with 

 certain types of ' knotenschiefer,' but respecting the thermal meta- 

 morphism of fine volcanic tuffs there seems to be little or no direct 

 information.' 



Lenticular inters tratifications of shale and mudstone make their 

 appearance even in the coarser parts of the conglomerate, as may be 

 observed on the 



beach below Canna Fig. 16. — Lava cutting out conglomerate and 

 House where, as shale. Shore below Canna House. 



shown in fig. 16, 

 some shales and 

 tuffs foil of ill- 

 defined leaves are 

 surmounted by 

 coarse conglome- 

 rate. The deposi- 

 tion of this over- 

 lying bed of 

 boulders has given 

 rise to some scoop- 

 ing-out of the finer 

 strata underneath. 



Subsequently both the conglomerate and the shales have been over- 

 spread by a stream of dolerite, the slaggy bottom of which has 

 ploughed its way through them. 



Before discussing the probable conditions under which the group 

 of sedimentary deposits now described was formed, we may con- 

 veniently follow the upper conglomerate-band of Compass Hill and 

 note the variations in structure and composition which its out- 

 crop presents. 



This yellowish conglomerate can be traced along the cliffs for 

 more than a mile, when it descends below the sea-level at the 

 solitary stack of Bod an Stol. A few hundred yards farther west, 

 what is probably the same band appears again at the base of the 

 precipice overlain by prismatic basalts. But the conglomerate, here 

 only 12 feet thick, is made of much finer detritus which, largely 

 composed of volcanic material, includes small, well-rounded and 

 polished pebbles of Torridon Sandstone. Beneath it lies a bed of 



1 Henslow, Trans. Camb. Phil. Soc. vol. i. (1822) p. 408 ; Harker, Geol. 

 Mag. 1887, p. 414. Mr. W. W. Watts suggests a comparison with the hexagonal 

 bodies figured by Mr. Monckton in an altered limestone from Stirlingshire, 

 Quart. Journ. Geol. Soc. vol li. (1895) p. 488. 



