112 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



far headlands of Skye to those of Antrim, it is obvious that they must be due to some 

 general cause, and that they contain the record of a special period or phase in the building 

 up of the volcanic tablelands. I will first describe some typical examples of them from 

 different districts, and then discuss their probable relations with the other portions of 

 the plateaux. 



First to be examined, and now most familiar to geologists, are the remarkable sheets 

 that underlie the plateau of Antrim, and project at various parts of the picturesque 

 line of coast from Portrush to Fair Head. From the shore at Portrush came the evidence 

 that was supposed to prove basalt to be a rock of aqueous origin, inasmuch as shells were 

 obtained there from what was believed to be undoubtedly basalt. The long controversy 

 to which this supposed discovery gave rise is one of the most curious in the history of 

 geology.* Fellows of this Society have cause to remember with pleasure that it was one 

 of their predecessors, the illustrious Playfair, who showed the pretended basalt to be in 

 reality highly indurated shale, and hence that, instead of furnishing proof of the aqueous 

 formation of basalt, the Portrush sections only contributed another strong confirmation of 

 the Huttonian theory, which claimed basalt to be a rock of igneous origin. 



It is now well known that the rock which yielded the fossils is a Liassic shale, that 

 it is traversed by several sheets of eruptive rock, and that by contact-metamorphism it 

 has been changed into a highly indurated substance, breaking with a splintery, conchoidal 

 fracture, but still retaining its ammonites and other fossils. The eruptive material is a 

 coarse, distinctly crystalline dolerite, in some parts of which the augite with its penetrating 

 lath-shaped crystals of plagioclase is remarkably fresh, while the olivine has begun to 

 show the serpentinous change along its cracks, t This rock has been thrust along the 

 bedding planes of the shales, but also breaks across them, and occurs in several sheets, 

 though these may all be portions of one subterranean mass. Some of the sheets are only 

 a few inches thick, and might at first be mistaken for sedimentary alternations in the 

 shale. But their mode of weathering soon enables the observer readily to distinguish 

 them. It is to be noticed that these thin layers of eruptive material assume a fine grain, 

 and resemble the ordinary dykes of the district. This closeness of texture, as Griffith 

 long ago pointed out, J is also to be noticed along the marginal portions of the thicker 

 sheets, where they lie upon or are covered by the shales. But away from the surfaces of 

 contact, the rock assumes a coarser grain, insomuch that in its thickest mass it presents 

 crystals measuring sometimes an inch in length, and then externally resembles a gabbro. 

 A more curious structure is shown in one of these coarsely crystalline portions by the 

 occurrence of a band a few inches broad which is strongly amygdaloidal, the cells, some- 

 times three inches or more in diameter, being filled with zeolites. § The general dip of 



* For an excellent summary of it and an epitome of the descriptions of the Portrush section, see the Report on the 

 Geology of Londonderry, &c, by J. E. Portlock (1843), p. 37. 



t Dr F. Hatch, Explanation of Sheets 7 and 8, Geol. Survey of Ireland, p. 40. 



X "Address to Geological Society of Dublin, 1835," p. 13, Jour. Geol. Soc. Dublin, vol. i. The varieties of the 

 Portrush rock were described by the late Dr Oldham, in Portlock's Report on the Geology of Londonderry, p. 150; see 

 also the same work for Portlock's own remarks, p. 97. 



§ For a list of the minerals in this rock, see Oldham, op. tit., p. 151. 



