Vol. 65.] THE CAULDEON-SUBSIDENCE'OE GLBN COE. 675 



would find in this great tube a plastic bod}-, of large cross-section, 

 incapable of resisting deformation. Its weakness would be 

 tbe dominating subterranean feature of the whole 

 district; its yielding a sufficient cause to locate the 

 two swarms of dykes. 



The effect of the pressure of the still molten contents of the tube 

 must not be lost sight of. The material of the dykes was probably 

 injected from the tube, and not directly from the molten reservoir 

 beneath. If the fissures had actually extended down to a general 

 underlying magma-basin, one might have expected to find them 

 frequently accompanied by faulting due to differential sagging, but : 

 such is not the case. 



In the foregoing pages the volcanic, plutonic, and hypabyssal 

 rocks of Glen Coe have been discussed in the light of a single 

 general principle, namely, the upward movement of igneous magma 

 in correlation with complementary subsidence of portions of the 

 earth's crust. 



There are good reasons for believing that, during Lower Old lied 

 Sandstone times, a widespread magma-basin extended at great' 

 depth beneath a tract of the earth's surface of which the Scotland 

 of to-day is but a portion. The condition was one of unstable 

 equilibrium, in so far as the magma was lighter than the 

 superincumbent mass of solid rock. Subsidence was the natural 

 response; and where the distribution of tangential stress in the 

 earth's crust was uniform, and where no earlier dominant structure 

 complicated the issue, the subsidence affected circumscribed areas. 

 The cauldron of Glen Coe clearly demonstrates the> 

 existence of conditions so symmetrical that no one 

 direction of faulting was more favoured than another. 



It is improbable that the cauldron of Glen Coe stood alone in its 

 own day. "We have already indicated that the neighbouring Etive 

 boss may be interpreted as an example of a subsidence of closely 

 analogous type, to wit, a subterranean cauldron ; and recently 

 evidence has been obtained that, half-a-dozen miles away to the 

 north, the igneous centre of Ben Nevis was pursuing a similar; 

 course of development. We may suppose that analogous areas of : 

 depression, large and small, were distributed at intervals over the 

 whole region which served as roof to the magma-basin of the 

 period. In fact, the dyke-phase of Etive strongly suggests 

 the growth of one such subsidence in the neighbourhood, a 

 subsidence so vast, so oceanic in scale, that it infringed upon the 

 independence of the smaller local centres. Eor a time the Etive 

 focus became a peripheral dyke-injector, its development governed 

 by that of its great companion. During this stage, while its sphere 

 of activity was notably enlarged, its individuality was still retained 5 

 and subsequently, when the sag towards the neighbouring basin 

 had been compensated and conditions of symmetry had been once 



