DURING THE TERTIARY PERIOD IN" THE BRITISH ISLES. 171 



of an ancient volcanic dome that had been erupted and worn down before the outflow of 

 the basalts which gradually accumulated around and over it. # Had this been the true 

 history of the locality, it is inconceivable that of a rock which decays so rapidly as this 

 trachyte, and strews its slopes with such abundance of detritus, not a single fragment 

 should occur between the successive beds of basalt which are supposed to have surrounded 

 and buried it. Though the several beds of basalt are well exposed all round, I could not 

 find a trace of any trachytic fragments between them, nor has Mr Symes, who mapped 

 the ground in detail for the Geological Survey, been more successful. The basalts near 

 the trachyte are hard and splintery, but not so distinctly altered as round the granophyre 

 in the Inner Hebrides. Were there no other evidence than that furnished by this 

 Antrim locality regarding the relation of the acid and basic rocks of the Tertiary volcanic 

 series of Britain, the question, I admit, could not be satisfactorily settled. But when we 

 compare the Antrim sections with those of the west of Scotland, particularly with those 

 of Mull (figs. 43, 46), we see their close resemblance, and can hardly hesitate to regard 

 the Irish trachyte as later than the basalts around it. For my own part, I have little 

 doubt that the trachytes and pitchstones are not only far younger than the plateau-basalts, 

 but are even later than the granophyres.t 



Fig. 57. — Section across the southern slope of Camearny Hill, Antrim, aaa, bedded basalts; b, trachyte. 



Besides this largest boss of trachyte a number of smaller knobs of trachytic, rhyolitic, 

 perlitic, and vitreous rocks appear at intervals both to the north and south of the Tar- 

 dree mass, entirely surrounded by basalt. Most of these lie within the area of the lower 

 group of basalts, but one of them to the south-east of Ballymena appears to cross from 

 that group into the upper basalts. These scattered patches, I have no hesitation in 

 believing to be intrusive bosses, sheets, or veins, which, like those of Mull and Skye, have 

 been injected into the basalts long after these rocks had been built up into the plateaux. 



* For an early account of the Antrim trachytic rocks, see Berger, Trans. Geol. Soc, iii. (1816) p. 190. Professor 

 Hull has described the Tardree rock in the Explanation to Sheets 21, 28, and 29, Geol. Survey of Ireland (1876), p. 17, 

 and has supposed it to be older than the basalts, referring it to the Eocene period. Dupfin (quoted by Mr Kinahan) 

 believed that " the trachytes occur at the centres of eruption, and were probably poured out at the end of the outburst." 

 Du Noyer also (quoted by the same writer) thought them to be newer than the plateau-basalts, and to have lifted up 

 masses of these rocks. Mr Kinahan himself (Geology of Ireland, p. 172) has pointed out the absence of any trachytic 

 fragments between the basalts as an argument against the supposed antiquity of the acid protrusions. A full petro- 

 graphical account of the Tardree rock is given by Von Lasaulx in the paper already cited, Tschermak's Min. Pet. 

 Mittheil, 1878, p. 412. 



t [Since this paper was read, and as it is passing through the press, I have received from Mr A. M'Henrt, of the 

 Geological Survey, some interesting information recently obtained by him at Templepatrick, co. Antrim. He has 

 there found what he considers to be conclusive evidence that the trachyte is intrusive in the Lower Basalts ; but that it 

 is pierced by younger basic dykes. This is precisely the structure which my experience in the Inner Hebrides would 

 have led me to expect.] 



