238 
THE TERTIARY VOLCANOES 
BOOK VIII 
contrast to the green terraced slopes of the plateau-basalts on which it rests, 
consists of some very distinct bands of black and grey lava, long known as 
“ pitehstone-porphyry.” To the nature and history of these rocks I shall 
return after we have considered a remarkable bed of conglomerate which lies 
below them. On the lower or southern side of the ridge the bottom of the 
pitchstone, dipping into the hill, is exposed on the roof of a small cave where 
the ends of its columns form a polygonal reticulation. It is there seen to 
repose upon a bed of breccia or conglomerate, having a pale-yellow or grey 
felspathic matrix like the more decomposing parts of the grey devitrified 
parts of the pitchstone. Through this deposit are dispersed great numbers 
of angular and subangular pieces of pitchstone, some of which have a striped 
texture. Fragments of basalt, red (Torridon) sandstone, and other rocks are 
rare ; and the bed suggests the idea that it is a kind of brecciated base or 
floor of the main pitchstone mass. A similar rock is found along the bottom 
of the pitchstone on both sides of the ridge (c, in Fig. 279). Here and there 
where this breccia is only a yard or two in thickness, it consists of sub- 
angular fragments of the various dolerites and basalts of the neighbourhood, 
together with pieces of red sandstone, quartzite, clay-slate, etc. The matrix 
is in some places a mass of hard basalt debris ; in others it becomes more 
calcareous, passing into a sandstone or grit in which chips and angular or 
irregular-shaped pieces of coniferous wood are abundant. 1 A little further 
east, beyond the base of the Scuir, a patch of similar breccia is seen, but 
with the stones much more rounded and smoothed. This outlier rests 
against- the denuded ends of the basalt-beds forming the side of the hill. 
Its interest arises from the evidence it affords of the prolongation of the 
deposit eastward, and consequently of the former extension of the precipice 
of the Scuir considerably beyond its present front. 
It is at the extreme north-western extremity of the pitchstone ridge, 
however, that the most remarkable exposure of this intercalated detrital 
band is now to be. seen. Sweeping along the crest of the plateau the ridge 
reaches the edge of the great precipice of Bideann Boidheach, by which its 
1 The microscopic structure of this wood was briefly described by Witham (Fossil Vegetables, 
p. 37), and two magnified representations were given to show its coniferous character. Lindley 
and Hutton further described it in their “Fossil Flora,” naming it Pinites eiggensis, and regard- 
ing it as belonging to the Oolitic series of the Hebrides — an inference founded perhaps on the 
erroneous statement of Witham to that effect. William Nicol corrected that statement by 
showing that the wood-fragments occurred, not among the “ Has rocks,” but “among the debris 
of the pitchstone” ( Edin . New Phil. Journal, xviii. p. 154). Hay Cunningham, in the paper 
already cited, states that the fossil wood really lies in the pitchstone itself! The actual position 
of the wood, however, in the breccia and conglomerates underlying the pitchstone is beyond all 
dispute. I have myself dug it out of the bed. The geological horizon assigned to this conifer, 
on account of its supposed occurrence among Oolitic rocks, being founded on error, no greater 
weight can be attached to the identification of the plant with an Oolitic species. Our knowledge 
of the specific varieties of the microscopic structure of ancient vegetation is hardly precise enough 
to warrant us in definitely fixing the horizon of a plant merely from the examination of the 
minute texture of a fragment of its wood. From the internal organization of the Eigg pine, there 
is no evidence that the fossil is of Jurassic age. From the. position of the wood above the 
dolerites and underneath the pitchstone of the Scuir it is absolutely certain that the plant is not 
of Jurassic but of Tertiary date. 
