CHAP. XLII 
THE BASIC SILLS OF ANTRIM 
299 
From the shore at Portrush, as I have already remarked, 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 . basalt. ^ The 
long controversy to which this supposed discovery gave rise is one of the 
most curious in the history of geology . 1 * It continued even after the 
illustrious Playfair had shown that the pretended basalt was 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, conelioidal fracture, but still retain- 
ing its ammonites and other fossils. The eruptive material is a coarse, 
distinctly crystalline dolerite, in some parts of which the augite, penetrated 
by lath-shaped crystals of plagioclase, is remarkably fresh, while the olivine 
has begun to show the serpentinous change along its cracks.' This rock 
lias been thrust between 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 , 3 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, sometimes three inches or more in diameter, 
• being 0 filled with zeolites . 4 The general dip of 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 promon- 
tory 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 
1 For an excellent {summary of this controversy and an epitome of the descriptions of the 
Portrush section, see the Report on the Geology of Londonderry, etc. (Mem. Geol. Survey), by 
J. E. Portlock (1843), p. 37. , , 
^ Dr. F. Hatch, Explanation of Sheets 7 and 8, Geol. Survey of Ireland, p. 40. 
•1 “ Address to Geological Society of Dublin, 1835,” p. 13, Jour. Geol. Soc. Dublin, vol. 1. 
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. 9 <■ 
4 For a list of the minerals in this rook, see Oldham, op. cit. p. 151. 
