LAKES IN RELATION TO GEOLOGICAL FEATURES 457 
Mr Harker in Skye have shown that the volcanic sequence there con- 
sists almost wholly of lavas of basic and sub-basic composition, except 
on the northern border of the Cuillins, where acid lavas and tuffs have 
been recorded. In places at the base of the volcanic series there are 
local accumulations of agglomerates and tuffs, but these are of rare 
occurrence in the great succession of lavas. 
Before the outbreak of vulcanicity the strata of pre-Tertiary age 
were subjected to earth-movements and extensive denudation. Hence 
the volcanic rocks rest, in places, on different members of the Jurassic 
and older systems, and in other localities on Upper Cretaceous strata. 
The volcanic rocks are pierced by masses of coarse gabbro typically 
represented in the Cuillin Hills, and by bosses of granophjre which 
form the well-known conical mountains between Sligachan and 
Broadford. These plutonic rocks likewise appear in Rum, Ardna- 
murchan. Mull, and St Kilda. Another characteristic feature of the 
period is the injection of sills of dolerite, which in places pass trans- 
gressively from one horizon of the volcanic sequence to another. But 
perhaps the most remarkable feature outside the volcanic plateaux is 
the extraordinary number of basalt dykes tilling rents or fissures, 
extending for miles across the surface of the country. They have 
been traced from Yorkshire to the West Highlands, and they stretch 
over an area rano-inff from the north of Ireland to the north of 
Scotland and probably to Orkney. According to Professor Judd, 
the lavas were discharged from great central volcanoes, the relics of 
which are now represented by the plutonic rocks of Skye, Rum, Mull, 
Ardnamurchan, and St Kilda. On the other hand. Sir A. Geikie 
contends that the lavas issued from innumerable fissures, a view which 
has been adopted by Mr Harker in the Geological Survey memoir 
on " The Tertiary Igneous Rocks of Skye.'" 
The only organic evidence bearing on the age of the great basaltic 
plateaux is supplied by the leaf beds occurring between the lava-flows 
at Ardtun in Mull. The researches of Mr Starkie Gardner point 
to the conclusion that the flora may be of Eocene age, and hence the 
extrusion of the volcanic rocks may belong to the earliest division of 
Tertiary time. 
Evolution of the Topographical Features of Scotland 
IN Pre-Glacial Time 
A careful study of Scottish topogi'aphy, especially of the river 
systems, leads to the theoretical conclusion that the existing features 
have been carved out of a solid block, the upper surface of which must 
have sloped towards the south-east, as suggested by Mr Mackinder 
in his volume on " Britain and the British Seas."" 
