318 



stone quarries, near Belfast, and at the Carlingford limestone quarries ; 

 they are sometimes composed of a mass of trap of one kind. The trap in 

 every dyke is modified in grain, according to the time occupied in cooling. 

 Narrow dykes are the finest grained ; they are often composed of pitch- 

 stone, or of rock closely allied to it. The cooling itself was also probably 

 modified by the temperature of the rock into which the melted trap was 

 injected. Dykes sometimes show a material difference between their 

 middle parts and their sides, both in composition and colour. The 

 change, too, is not gradual, but in steps, each step being like a separate 

 wall, and remarkably persistent in its width. Some dykes are composed 

 of three, or four, or five such divisions. Those walls appear each to 

 have been a separate projection, and one to have been cooled and 

 hardened before a second was injected, the whole forming a compound 

 dyke. After the first fissure was made, filled, and hardened, new sub- 

 terranean force was generated below, and new fluid matter made ready 

 to be protruded. The side of an old fissure was again more easily pene- 

 trated than a new one opened, perhaps through some miles in thickness 

 of rock. 



Dr. Eichardson has drawn up a careful account of fourteen whin 

 dykes on the north coast of Antrim, between Portrush and Port Coon, 

 which is printed in Dubourdieu's Statistical Survey of the county Antrim, 

 p. 68, Appendix. Whoever follows Dr. Eichardson can add but little to 

 his clear and accurate descriptions. He found one dyke at the Giant's 

 Causeway twenty feet thick ; one at Port na Spania, twelve feet. At 

 the west end of the white rocks, near Portrush, he saw one an inch and 

 a-half wide ; another only half an inch. All the dykes on the coast are 

 between these extremes of thickness, and the usual range is from three 

 to twelve feet. Vertical whin dykes, which are the usual kind here 

 are mostly composed of horizontal prisms or columns. These prisms 

 are sometimes three or four feet in diameter ; and the thick prisms 

 are again subdivided into smaller ones of three or four inches in diameter, 

 or one inch, or half an inch. 



In the " Transactions of the Geological Society of London," vol. 

 iii., Dr. Berger gives some features of the whin dykes in the Ballycastle 

 collieries, which I have partly described in the account of the coal- 

 measures, p. 246. Those are chiefly — l,the Saltpans dyke is 8 yards wide; 

 2, the north star dyke is 8 yards wide ; it has often been cut through 

 in working the collieries ; it does not shift the coal, but has reduced it 

 to cinders for 9 feet on each side; 3, Carrickmore dyke rises 30 feet 

 over water; it is about 12 feet wide, but irregular. The rocks in con- 

 tact with it are black shale on one side, and white sandstone on the 

 other, showing a downthrow of the strata. These rocks are altered at 

 the contact — the black shale into flinty slate, and the sandstone changed 

 from red to white. At 1 5 yards from the dyke the alteration ceases. 

 Within the colliery the coal is altered to cinders, and was only used for 

 burning lime. There are other unimportant thinner whin dykes here, 

 and some slips, which throw the strata up or down. These shifts are 

 described in the account of the coal-measures. 



