62 



DR GETKIE ON THE HISTORY OF VOLCANIC ACTION 



IIP 



Fig. 16. — Intersection of Dykes in bedded 

 basalt, Caliach Point, Mull. 



ated from that which it has invaded. Its rock is generally rather fine-grained, some- 

 times decidedly porphyritic, and in many cases is a true basalt. Where it is broad 



enough to show the difference between margin and 

 centre, its outer edges present the usual finer grain. 

 There can be no doubt that the older dyke has been 

 actually split open, and fresh eruptive material has 

 risen along the fissure. 



If the subterranean movements were energetic 

 enough to split up an already consolidated dyke, so as 

 to allow of the renewed uprise of molten material between 

 the separated portions, we may believe that much more 

 frequently the opening would be effected not along the 

 middle of the dyke, but between the dyke and one of its 

 fissure walls. I have observed examples of this structure 

 in the Western Islands, and Mr Clottgh has recently 

 found a number in Argyleshire. If the section is limited 

 in extent, we may be unable to determine which is the 

 older of the two parallel bands of igneous rock, though 

 the fact that they present to each other the usual fine- 

 grained edge due to more rapid cooling, shows that they are not one but two dykes 

 belonging to distinct eruptions. So far as I have noticed, where one of the dykes can 

 be continuously traced for a considerable distance, the other is short, and cannot be 

 recognised at other exposures of the more continuous one. I infer that the shorter 

 one is the younger of the two. 



§ 15. Contact-Metamorphism of the Dykes. 



Another anticipation which a geologist might naturally entertain is, that these 

 abundant intrusions of igneous rock should be accompanied with plentiful evidence 

 of contact-metamorphism along their flanks. But in actual fact, evidence of any serious 

 amount of alteration is singularly scarce. A slight induration of the rocks on either side 

 of a dyke is generally all the change that can be detected. 



Some of the larger dykes, however, show more marked metamorphism, the nature of 

 which is chiefly determined by the chemical composition of the rock affected. The most 

 pronounced alteration is that which has been superinduced on carbonaceous strata, and 

 particularly on seams of coal. In the Ayrshire coal-field the alteration of the coal extends 

 sometimes 150 feet from the dyke, the extent of the change depending not merely on 

 the mass of the igneous rock, but on the nature of the coal, and possibly on other causes. 

 Close to a dyke, coal passes into a kind of soot or cinder, but sometimes assumes the 

 form of a finely columnar coke.* Shales are converted into a hard flinty substance that 

 breaks with a conchoidal fracture and rings under the hammer. Fire-clay is baked into a 



* Explanation of Sheet 22, Geol. Survey Scotland, p. 26. 



