674 MESSRS. CLOUGH, MATTFE, AND BAILET ON [ISoV. 1909, 



accompanied by a conspicuous extension of the whole district in a 

 direction normal to their trend. The regional stress thus indicated 

 must clearly have been of the nature of relative tension, or in 

 other words of marked relief of pressure. This, alike assisting and 

 controlling the further introduction of the igneous magma, led to 

 the formation of dykes orientated at right angles to the direction 

 of least compression. 



Since the dykes are parallel, the growth of tension in the district 

 must have been repeatedly in the one direction throughout a 

 protracted period. The dykes represent the intermittent response 

 to the growth of the regional stress, and their moderate breadth 

 indicates that, in each case, the tensional earth-stress co-operating 

 with the pressure of the magma was accommodated by a moderate 

 displacement of the walls of the fissure. 



Following Marcel Bertrand l and Harker 2 we are inclined to 

 seek the cause of this long-continued growth of tension 

 in some widespread contemporaneous depression of 

 a neighbouring portion of the earth's crust. A re- 

 distribution of stresses must accompany such a depression ; an 

 extensive peripheral region must be involved ; and masses which 

 have long been stagnant must stretch and yield, in accordance 

 with the new conditions of equilibrium. VieAved in this light, 

 the dyke-fissures of the Lome and Glen Coe districts have a distinct 

 analogy with the transverse crevasses of a glacier. An expla- 

 nation is thus afforded of parallel dykes scattered throughout a 

 large block of country. 



If the constitution of the district affected by the regional stress 

 be homogeneous, then the resultant distribution of the dykes 

 should theoretically be uniform. If, on the other hand, there be 

 local differences in the capacity of adjacent rock-masses to resist 

 the stress, then the distribution of the dykes will be correspondingly 

 modified. 



In the light of this conception, the local concentration of the 

 porphyrite dykes in relation to the Etive centre can be interpreted 

 Avith some degree of probability. We have already suggested that 

 beneath the granite exposed at the surface, there extends a tube 

 largely filled with plutonic rocks, which reaches down to the 

 general magma-basin below. Probably the contents of the tube at 

 no great distance doAvn were still molten during the dyke-phase. 

 Indeed, the intrusion of the acid granitic strip between Stob Ban 

 and Meall Odhar intervened in the early portion of this period ; 

 Avhile, at a stage which cannot definitely be fixed, but was, 

 perhaps, entirely subsequent to the dyke-phase of activity, the 

 Starav Granite rose into its present position, so as to form the core 

 of the Etive boss at the level now laid bare by denudation. The 

 regional stresses, tending by their action to produce extension, 



1 ' Sur la Distribution Geographique des Roches Eruptives en Europe ' Bull. 

 Soc. Geol. France, ser. 3, vol. xvi (1888) pp. 573-617. 



2 ' The Tertiary Igneous Rocks of Skye ' Mem. Geol. Snrv. 1904, chap. xxv. 



