DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 99 



which are repeated on cliff after cliff, may be considered typical for all the plateaux. 

 Another characteristic point, well displayed here, is the intervening red parting between 

 the successive beds. If the occurrence and thickness of this layer could be assumed 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 of the earthy amygdaloidal sheets ; but 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 subserial decay than those of the compact varieties. 

 Nevertheless, I 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 

 M'Leod's Maiden, where the red layers are very prominent, were poured out at longer 

 intervals than those that form the upper half. 



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 hill-side to hill-side. 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, so 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 Rum, Ardnamurchan, and Mull. Portions 

 of it which have survived indicate with sufficient clearness that it once spread south- 

 wards 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 Eed Hills and the Cuillins, to whose picturesque forms Skye owes so 

 much of .its charm. 



In this, as in each of the other plateaux, there is no trace of any thickening out 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 characters ; they do not become thicker there either 

 collectively or individually, nor are they more abundantly interstratified with tuffs or 

 volcanic conglomerates. 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. 



