DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



113 



the shales and of the intrusive sheets which have been injected between them is towards 

 the east. From underneath them a thick mass of dolerite rises up to form the long 

 promontory that here projects northwards from the coast-line, and is prolonged seawards 

 in the chain of the Skerries. 



An interesting feature of the Portrush sections is the clear way in which they exhibit 

 the phenomena of segregation-veins — so characteristic of the thicker and more coarsely 

 crystalline intrusive sheets. These veins or seams differ from the rest of the rock mainly 

 in the much larger size and more definitely crystalline form of their component minerals. 

 Though sharply defined, when looked at from a little distance, they are found on closer 

 inspection to shade into the surrounding rock b}^ a complete interlacing of crystals. 

 On the shore, they can be seen to lie on the whole parallel with the bedding of the sheets 

 in which they occur, but without rigidly following it, since they undulate and even 

 ramify. A good section across their dip has been exposed in a quarry near the end of 

 the promontory, and shows that they are considerably less regular than the plan of their 



Fig. 32.— View of Segregation Veins in dolerite of an Intrusive Sheet, Portrush, Antrim. 



outcrop on the shore would have led us to anticipate. The accompanying drawing (fig. 

 32) represents the veins laid bare on a face of rock 9 feet in length by 5 feet in height. 

 It will be seen that while there is a general tendency to conform to the dip-slope, which, 

 is here from right to left, the seams or layers unite into a large rudely bedded mass, 

 which sends out processes across the bedding. The peculiar aggregation of minerals 

 which distinguishes such segregation veins is perhaps best seen at Fair Head, and I 

 reserve for the description of that locality what I have to say on the subject, only 

 remarking with regard to the Portrush rock that the felspar shows a disposition to collect 

 in the centre of the veins with the augite and the other dark minerals at the outer 

 margins. 



The contact-metamorphism at this locality is of more historical interest in connection 

 with the progress of geological theory than of scientific importance. It consists mainly 

 in an intense induration of the argillaceous strata. These pass here from their usual 



