CHAP. XXXVIII 
RIVERS OF THE PERIOD 
245 
of the bed lie as kernels in the midst of the more lithoid and decom- 
posed rock. The lower six feet of the ‘'porphyry” are white and still more 
decomposed. The relations of this mass are represented in Fig. 282, where 
the basalt-rocks of the plateau (a) are shown to be cut through by basalt 
dykes (5 V), and overlain by the “ porphyry ” (c) and the pitclistone (d). In 
the porphyry are shown several pitchstone kernels It is deserving of 
remark also that in different parts of the Scuir, particularly along the north 
side, the bottom of the pitchstone beds passes into a dull grey earthy lithoid 
substance, like that now under description. 
The bedded character of the rock of the Scuir and the well-marked 
lithological distinction between its several component sheets show the lava 
to have been the product of a number of separate outflows that found their 
way one after another into the river- valley, which was the lowest ground in 
the vicinity of the active vent. There can be little doubt, I think, that the 
lava flowed down the valley. Its successive streams are still inclined from 
east to west. The vent of eruption, therefore, ought to be looked for towards 
the east. Nowhere within the Tertiary volcanic region is there any boss of 
pitchstone or any mass the shape or size of which is suggestive of this 
vent. In the island of Eigg no boss of any kind exists, save those of 
granophyric porphyry to be afterwards referred to. But none of these 
affords any satisfactory links of connection with the rock of the Scuir. 
More probably the vent lay somewhere to the east 011 ground now overflowed 
by the sea. The pitchstone veins of Eigg may represent some of the sub- 
terranean extrusions from the same volcanic pipe, and if so, its site could 
not be far off. 
The rock of the Scuir of Eigg has a special importance in the history 
of the volcanic plateaux. It is, so far as we know, the latest of all the 
superficial lavas of Britain. 1 From the basalts on which it rests it was 
separated by an enormous interval of time, during which these older lavas 
were traversed by dykes and were worn down into valleys. Its presence 
shows that long after the basalts of Small Isles had ceased to be erupted, a 
new outbreak of volcanic activity took place in this district, when lavas of a 
more acid composition flowed out at the surface. Whether this outburst 
was synchronous with the appearance of the great granophyric protrusions 
of the Inner Hebrides, or with the still later extravasation of pitchstone 
dykes, can only lie surmised. 
When one scans the great precipice on the west side of Eigg, with its 
transverse section of the pitchstone-lava, buried river-bed and basalt- 
plateau underneath, there seems no chance of any further westward trace ot 
the pitchstone being ever found. The truncated end of the Scuir looks 
from the top of the cliff out to sea, aud the progress of denudation might 
1 The rocks of Beinn Hiant in Ardnamurchan have been claimed by Professor Judd as super- 
ficial lavas. For reasons to be afterwards given (p. 318) I regard them as intrusive sheets. 
Professor Cole believes the rhyolites and pitchstones of Tardree to be probably evidence of a 
volcano later than the basalts of Antrim. As 1 have not been able to detect any actual proofs of 
superficial outflow there, 1 relegate the description of the rocks to a future chapter, in which 
the acid protrusions will lie discussed (p. 426). 
