368 



SIE A. GEIKIE ON THE TEETIAEY 



[May 1896, 



front of the stack is shown in PI. XYIII. If, however, we approach 

 the rock from the coast-gully to the north, we form a very different 

 impression of its structure. It then appears to consist chiefly of 

 conglomerates with a capping of basalt on the top. It is not until 

 a close scrutiny is made of the eastern and western faces of the 

 column that the true structure and history of this singular and 

 striking piece of topography become apparent. 



On the eastern front the section represented in fig. 18, p. 367, is 

 exposed. At the bottom, forming the pediment of the column, lies a 

 sheet of slaggy and vesicular or amygdaloidal basalt (a), which shelves 

 gently in a south-westerly direction into the sea. The lowest band 

 (b) in the structure of the stack is a thin group of lilac, brown, and 

 green shale and volcanic mudstone or tuff, which encloses pieces of 

 coniferous wood, and becomes markedly carbonaceous in its upper- 

 most layers. Above these strata on the southern front comes the 

 pile of bedded basalts (c) with their slaggy lower and upper surfaces. 

 But as we follow them round the eastern side we find them abruptly 

 cut off by a mass of conglomerate (d). That the vertical junction- 

 line is not a fault is speedily ascertained. The lower platform of 

 slaggy basalt runs on unbroken under both the shales and the con- 

 glomerate. Moreover, the line of meeting of this conglomerate with 

 the basalts that overlie the shales is not a clean-cut straight wall, 

 but displays projections and recesses of the igneous rocks round and 

 into which the materials of the conglomerate have been deposited. 

 The pebbles may be seen filling up little crevices, passing under over- 

 hanging ledges of the basalts, and sharply truncating lines of scoria- 

 ceous structure in these rocks. The same relations may be observed 

 on the western front of the stack. There the ashy shales and tuffs 

 are sharply cut out by the conglomerate which wraps round and 

 underlies a projecting cornice 



of the slaggy bottom of the Fig. 19. — Enlarged section on the 

 basalt that rests on the strati- western side of the Dun Beag. 

 fied band (fig. 19). 



The conglomerate is rudely 

 stratified horizontally, its bed- 

 ding being best shown by 

 occasional partings of greenish 

 sandstone. It consists of well- 

 rounded, polished, and water- 

 worn stones, chiefly of mem- 

 bers of the volcanic series, — 

 basalts, and dolerites, both 

 compact and amygdaloidal or 

 slaggy, — but with a conspicuous 

 admixture of Torridon Sand- 

 stone, gneiss, grey granite, grit, 

 and different schists. The 

 coarsest part of the deposit 



lies towards the bottom, where the volcanic blocks are some- 

 times 6 and 8 feet in diameter. Some of these large masses 



a, amygdaloid ; b, tuff ; c, ashy shales ; 

 d, layer of coaly shale; e, amyg- 

 daloidal basalts ; /, conglomerate. 



