Vol. 65.] 



THE CATJLDEON-STJBSIDENCE OF SEES' COE. 



643 



Besides, as it were, setting their seal upon Glen Coe, and thus 

 establishing the fact that all its remarkable history of movement 

 was accomplished in one long period of igneous activity, these dykes 

 by their mode of intrusion bear witness to a redistribution of earth- 

 stresses, which took place after all subsidence had ceased, and brought 

 about an important modification of the original outline of the 

 cauldron, amounting to an elongation normal to the direction of 

 the dykes. 



In many clear exposures the cheeks of the dykes are strictly 

 parallel, and this parallelism is usually maintained, even in places 

 where the dyke shifts itself laterally in its course. In the accom- 

 panying diagram (fig. 3, p. 642) of a porphyrite dyke traversing 

 rhyolite at the foot of the northern front of the Buachaille Etive 

 Beag, it will be seen that the cheeks of the dyke are counterparts 

 the one of the other, and that if the material of the dyke could 

 be removed, the walls of the country-rock might be closed up com- 

 pletely without any gap remaining. An apparent exception exists 

 at the angle marked X, where the eastern wall bends in sharply 

 in advance of the deflection of the opposite wall, in such a way 

 that a gap would be left, if the walls of country-rock were fitted 



together again. 



Pig. 4. — Porphyrite dyJce traversing the Moor of -° u ^ ] us ^ a ^' 

 Bannoch Granite in the bed of the River this point there 

 Etive, 500 yards above Kingshouse. (About floats in the 

 ir not. size.) d Y ke an mclu " 



sion of country- 

 rock, the size 

 and shape of 

 which is such 

 that it would 

 exactly fill the 

 gap. It should 

 be remarked 

 that the lateral 

 displacement of 

 the dyke is not 

 due to cross - 

 faulting, for the 

 dark chilled 

 border of the 

 dyke-rock fol- 

 lows round all 

 the irregular - 

 ities of the 



PORPHYRITE 



DYKE 



^ 



[Complementary portions of two basic secretions in the 

 granite are found on opposite sides of the dyke.] 



margin, and even surrounds the inclusion. Hence we may con- 

 clude that the dyke-magma was intruded into a fissure, opening 

 in the country-rock, and that no appreciable absorption of the 

 latter took place. 



This argument is clinched by an example taken from a dyke 



