'644 MESSES. CLOTTGH, MATTEE, AND BAILEY ON [Nov. I909, 



intruded into the Rannoch Granite, and exposed in the River Etive 

 500 yards ahove Kingshouse. The granite contains dark basic 

 secretions, ellipsoidal in shape, and rarely more than a foot in 

 length. Two of these secretions are truncated at the margin of the 

 dyke, and exactly opposite, on the further edge of the dyke, are 

 found their complementary portions (fig. 4, p. 643). The size and 

 shape of the separated parts of the secretions are such that there 

 can he no doubt that the parts were united before the intrusion of 

 the dyke. This example shows incidentally that the dyke was 

 not intruded along a line of fault, an observation which may be 

 verified again and again in the numerous cases of dykes intruded 

 into the bedded volcanic rocks. Reference has already been made 

 to the fact that dykes occasionally follow pre-existing fault-lines, 

 but we cannot point to any case in which the intrusion of a dyke 

 was accompanied by faulting. 



It is clear then that the dykes of Glen Coe are not 

 associated with phenomena of faulting or absorption of 

 the country-rock, but that their thickness is a definite 

 addition to the cross-section of the district which they traverse, 

 The fissures which they occupy are, perhaps, merely widened joints, 

 for the volcanic rocks are traversed by two sets of joints, one of 

 which trends north-north-east and south-south-west, while the other 

 runs at right angles to this direction. It is important to notice 

 in this connexion that the joints, like the dykes, run in parallel 

 lines, and maintain their general verticality even where they pass 

 through the tilted or overturned volcanic rocks on the margin of 

 the cauldron. Further, in areas in which the dykes are inclined 

 away from the vertical, as in the neighbourhood of Alltchaorunn, 

 the chief joints are also inclined in the same direction and to 

 about the same extent as the dykes. 



From the facts enumerated above we may conclude that the period 

 following on the final subsidence in the cauldron was marked by 

 the formation of a vertical, or nearly vertical, series of joints. 

 Subsequently, certain of the joints, trending north-north-east and 

 south-south-west, were opened and injected with molten material 

 under conditions of relative tension in the rocks traversed by the 

 dykes. This process of intrusion must have taken place inter- 

 mittently, for composite dykes, in which a later dyke has been 

 intruded along the central line or the margin of a previous one, 

 are not uncommon in the district. 



The tensional stresses, which, co-operating with the hydrostatic 

 pressure of the magma, opened the joints and admitted the dykes, 

 must have acted in a direction normal to that taken by these 

 intrusions. The amount of stretching, or rather the displacement 

 of two points at opposite ends of the district, might be measured by 

 the sum of the widths of the dykes. On enquiring into this matter 

 in detail, it was found from examination of the section of the River 

 Etive, a little above Alltchaorunn, that in a distance of 1133 yards, 



