SCIENCE-GOSSIP. 



253 



THE EURITE OE GLASDRUMMAN PORT. 

 By Prof. Grenvillk A. J. Cole, F.G.S., M.R.I. A. 



TV X K. WELCH'S photograph, which is repro- 

 duced on this page by his kind permission, 

 is one of three, illustrating a great composite 

 dyke on the east coast of Co. Down. This is 

 eminently an eruptive district. The uptilted Ordo- 

 vician (Lower Silurian) strata were broken through 

 by a series of fissures, into which basalts and allied 

 rocks oozed in a molten state, forming the immense 



mass of eurite forced its way, so that we now have 

 a dyke with four feet to seventeen feet of basaltic 

 andesite at its two sides, and thirty-six feet of pale 

 pinkish eurite in the middle. Numerous blocks 

 of the andesite were torn off and became included in 

 the eurite ; they weather away more easily than the 

 latter rock, and thus become sunk in the broad 

 wave-worn surface of the eurite. The photograph 



R. Welch. Photo.] 



Porphyritic Eurite of Glasdrumman Port, Co. Down. 

 (Arrows point to fragments of Basaltic Andesite.) 



[Belfast. 



number of dykes now to be seen upon the shore. 

 Somewhat later, an enormous intrusion of granite 

 occurred, forming the central mass of the Mourne 

 Mountains, and cutting off the earlier dykes all 

 round its margins. A few new dykes of fine- 

 grained granite (eurite) also arose, cutting across 

 the others, or intruding along them and forming 

 "composite dykes," like that of (llasdrumman. 

 Here there was originally a dyke of basaltic 

 andesite — a dark, fine-grained, and partly glassy 

 rock, containing labradorite felspar and brown 

 angite. Into a fissure in the centre of this, a broad 

 December, 1895.— No. 22, Vol. II. 



shows well the rounding of the eurite by wave- 

 action, and by the sand and pebbles thrown against 

 it at every high tide. Behind is the low cliff of 

 pebbly "drift" that fringes the coast of Mourne. 

 In front, some of the dark blocks of andesite 

 are seen included in the eurite, and are indi- 

 cated by arrows. The eurite is speckled over 

 with porphyritic crystals of felspar, especially 

 towards its margins. The phenomena of these 

 margins themselves have been the subject of a 

 recent paper (" Scientific Transactions of the Royal 

 Dublin Society," vol. v., page 239), since the most 



