LAKES IN RELATION TO GEOLOGICAL FEATURES 455 
In the West Highlands the Jurassic and Cretaceous strata occur 
at intervals over an area measuring about 120 miles from north to 
south and about 50 miles from east to west. Long after their 
formation they were buried under a succession of lavas and intrusive 
sills of igneous rock of Tertiary age. Round the edges of the volcanic 
plateaux, or where the streams have cut deep trenches through the 
overlying igneous rocks, the mesozoic strata are exposed anew. 
The researches of Professor Judd have shown that in the Jurassic 
rocks of Scotland there is a constant alternation of estuarine and 
marine strata, presenting considerable variations at different localities. 
For example, the estuarine sandstones, conglomerates, and shales at the 
base of the Lower Lias in Sutherland are overlain with micaceous 
clays and shelly limestones, with characteristic marine forms ; while 
the lower Oolite is composed mainly of estuarine strata, consisting of 
sandstones, shales, and coals. On this latter horizon the well-known 
coal seams are met with, one of them reaching a thickness of ^ feet. 
Again, in the Middle Oolite there are several important marine zones 
alternating with estuarine strata ; of the former, the " roof-bed of the 
Main Coal is an excellent example, consisting of sandstone passing 
into a limestone charged with ammonites and belemnites belonging 
to the horizon of the Kelloway rock in Yorkshire. Finally, the 
Upper Oolites are represented in Sutherland by a splendid develop- 
ment of sandstones, shales, grits, and brecciated conglomerates indica- 
ing estuarine conditions. 
In the West Highlands the same recurrence of estuarine and 
marine conditions is observable, though in a less prominent form. At 
certain localities sandstones and thin coal seams are to be found at 
the base of the Lias, while between the Lower Oolite and the Oxford 
Clay a great estuarine series is intercalated, consisting of sandstones, 
shales with much carbonaceous matter, and limestones made up of 
comminuted shells. According to Professor Judd, nearly 3000 feet of 
Jurassic strata are exposed in the West Highlands from the base of 
the liias to the Oxford Clay, to which must be added about 1000 
feet of beds representing the L^pper Oolite in the north-east of 
Scotland, so that the total thickness of these rocks cannot be far 
short of 4000 feet. 
CRETACEOUS 
Between the Jurassic and Cretaceous strata of the West Highlands 
there is a prominent unconformability, indicating striking changes in 
the physical geography of the region and extensive denudation of the 
deposits that had accumulated during previous periods. During this 
interval the floor of the sea must have been elevated and the Jurassic 
sediments must have been removed, either in whole or in part, from 
