220 Prof. Grenville Cole — The Tachylytes of Co. Doivn. 



This question of rock-metamorpliism is deserving of full con- 

 sideration. If I have not made myself clear to my critic, or to any 

 other of the readers of this Magazine, I shall be glad to give further 

 explanation ; and, if I am wrong, I will engage to surrender to 

 General McMahon and his legion of Eoman soldiers. 



YI. — On Vaeiolite and othek Tachylytes at Dunmore Head, 



Co. Down. 



By Grenville A. J. Cole, M.E.I.A., F.G.S. 



Professor of Geology in the Eoyal College of Science for Ireland. 



WHILE examining the dykes along the coast of Mourne in the 

 company of Mr. E. Welch, I came across an additional 

 example of intrusive variolite, the occurrence of which seems worth 

 recording. The mass is, in this case, a thin one, about 20 centi- 

 metres in width, narrowing and dying out as it is traced northwards; 

 it runs in a somewhat sinuous course along the strike of the uptilted 

 strata. Every other dyke that I have seen upon this richly favoured 

 coast cuts across the Ordovician beds, and the present instance to 

 the contrary may be only a lateral sheet-like offshoot from one of 

 the familiar basalts. 



This variolite occurs on Dunmore Head, just south-east of the 

 summit of Dunmore Hill, and a little below the level of high- 

 water. The handsome and massive variolite of Annalong ^ lies some 

 2 1 miles to the south. 



The thin intrusive sheet is minutely spherulitic upon its selvages, 

 but passes rapidly into a typical variolite, with spherulites 3 mm. 

 in diameter. These bodies weather out in pale tints against the 

 dark grey-green groundmass. The specific gravity of a large typical 

 specimen is 2-86, being nearly as high as that of the variolites of the 

 Western Alps (2-90), while the rock of Annalong yields only 2-72. 



In the example from Dunmore Head we have a variolite obviously 

 connected with ordinary tachylyte, and it seems unnecessary to 

 again urge the relationship between the two rocks, variolite being 

 merely an altered form of the more coarsely spherulitic types of 

 basic glass. Professor Zirkel,^ in preferring to regard the rock of 

 Annalong as a " sphaerolithfuhrendes Diabasglas " rather than as a 

 true variolite, practically admits the impossibility of maintaining 

 variolite as a distinct rock-species. The name is a convenient term 

 for specialists, like the corresponding word " pyromeride," in the 

 classification of highly silicated rocks ; and it has by far too gi'eat 

 an antiquarian interest to be thrust aside into oblivion. 



At Annalong the dyke is bounded by a film of tachylytic glass 

 6 mm. in thickness ; but the main mass is an indubitable variolite. 

 The rock of Dunmore Head has an altered groundmass, with traces 

 of glass on the extreme selvage ; but the spherulites are far more 

 fresh and interesting than in the variolites that I have hitherto 

 examined. Under the microscope they are seen to be composed, 



1 Proc. Eoy. Dublin Soc. vol. vii. (1892), p. 511. 



2 "Lehrbuch der Petrographie," 1^ Auliage (1894), 1'^" Band, p. 704. 



