CHAP. XXXIX 
THE PLATEAU OF SKYE 
255 
as an indication of the relative lapse of time between the different 
flows of lava, it would furnish us with a rude kind of chronometer for 
estimating the proportionate duration of the intervals between the eruptions. 
It is to be noticed on the top both of the compact prismatic and oi the 
earthy amygdaloidal sheets ; but it is more frequent and generally thicker 
on the latter than on the former, which may only mean that the surfaces of 
the cellular lavas were more prone to subasrial decay than those of the 
compact varieties. Nevertheless, 1 am disposed to attach some value to it, 
as an index of time. In the present instance, for example, it seems to me 
probable that the lavas in the lower half of Maeleod’s Maiden, where the red 
laYers are very prominent, were poured out at longer intervals than those 
that form the upper half. The remarkable banded arrangement of the 
vesicles in one of the cellular lavas of this sea-stack has been already 
referred to (p. 191). 
Another characteristic plateau-feature is admirably displayed in Skye — 
the flatness of the basalts and the continuity of their level terraces (though 
not of individual sheets) from cliff to cliff and hillside to hillside. This 
feature may be followed with almost tiresome monotony over the whole of 
the island, north of a line drawn from Loch Brittle to Loch Sligachan. 
Throughout that wide region, the regularity of the basalt-plateau is unbroken, 
except by minor protrusions of eruptive rock, which, as far as I have noticed, 
do not seriously affect the topography. But south of the line just indicated, 
the plateau undergoes the same remarkable change as in Lum, Aidnamui- 
chan and Mull. Portions of it which have survived indicate with sufficient 
clearness that it once spread southwards and eastwards over the mountainous 
district, and even farther south into the low parts of the island. Its removal 
from that tract has been of the utmost value to geological research, for 
some of the subterranean aspects of volcanism have thereby been revealed, 
which would otherwise have remained buried under the thick cover of basalt. 
Denudation has likewise cut deeply into the eruptive bosses, and has carved 
out of them the groups of the lied Hills and the Ouillins, to whose pictur- 
esque forms Skye owes so much of its charm. 
In this, as in each . of the other plateaux, there is no trace of any 
thickening of the basalts towards a supposed central vent of eruption. 
The nearly level sheets may be followed up to the very edge of the great 
mountainous tract of eruptive rocks, retaining all the way their usual charac- 
ters : they do not become thicker there either collectively or individually, nor 
are they more abundantly interstratiiied with tufts or volcanic conglomeiates. 
On the contrary, their very base is exposed around the mountain ground, and 
the thickest interstratifications of fragmentary materials are found at a 
distance from that area. So far as regards the structure of the remaining 
part of the plateau, the eruption of the gabbros and granitoid rocks might 
apparently have taken place as well anywhere further north. 
