170 
THE TERTIARY VOLCANOES 
BOOK VIII 
least a complete deflection of their course might he anticipated. It would 
require, we might suppose, a singularly potent dislocation to open a way for 
the ascent of the lava through such crushed and compressed rocks, and still 
Ixa. 257. Map of the- chief dykes between Lochs Riddon and Striven (C. T. Clough, Geological Survey 
Sheet 29). The large E. and W. dyke is a continuation of that which reaches the shore of the 
Firth of Clyde at Dunoon. 
more to prolong the general line of a fracture across the old fault. Two 
great dykes, about half a mile apart, run in a direction a little south of west 
across the plain of Strathearn. Passing to the south of the village of 
Crieff, they hold on their way until they reach the highly-inclined beds of 
sandstone and conglomerate which here lean against the Highland fault in 
