Vol. 54.] THE BALA. & OTHER ROCKS OF LA.MBA.Y. 135 



9. The Bala. Beds and Associated laNEOTis Rocks of Lambay 



Island, Co. Dublin. By C. I. Gardiner, Esq., M.A., F.G.S., 



and S. H. Reynolds, Esq., M.A., E.G.S. (Read December 1st, 



1897.) 



[Plate IX— Map.] 



Contents. 



Page 



I. Introduction 135 



II. The Sedimentary Rocks 13o 



III. The Igneous Rocks 140 



(a) The Fragmental Rocks 140 



(6) The Andesitic Rocks 142 



(c) The Coarse Porphy rite 145 



IV. Conclusions 147 



I. Introduction. 



Lambay Island lies off the East Coast of Ireland, some 10 miles 

 north of Dublin, the nearest point on the mainland being in the 

 district of Portraine, where there occurs an inlier of Bala rocks 

 which we have already described.^ 



The name of Lambay is probably familiar to geologists from the 

 occurrence there of the ' Lambay porphyry,' a very handsome and 

 striking rock, described in detail by Von Lasaulx ^ and mentioned 

 in many geological and petrological works.^ 



It was with the intention of investigating the occurrence of 

 this rock in the field, and at the same time of seeing whether the 

 other rocks exposed on the island were of a similar nature to those 

 at Portraine, that we visited the island. 



The only detailed account of the geology of Lambay appears to 

 be in the explanatory memoir to Sheet 102 of the Geological Map 

 of Ireland, published in 1861. Homogeneous and porphyritic trap- 

 rocks are mentioned, catching up in places masses of slates and grits 

 which they have baked and hardened. Ashes were also found, 

 while limestone and graptolitic shales gave evidence that some 

 of the rocks of the island were of Bala age. 



On looking at the map of the island it will be seen that a 

 large part of it is drift-covered, but from the exposures which 

 occur it is obvious that the greater part of Lambay is formed of 

 igneous rocks, the sedimeutaries occurring in detached masses of 

 no very great extent, and, except in the case of the limestone, they 

 yield little fossil evidence from which their age can be determined. 

 It will be most convenient to describe first the sedimentary and 

 then the igneous rocks. 



1 Quart. Journ. Geol. Soc. vol. liii (1897) p. 520. 



2 Tscherm. Min. u. Petrogr, Mitth. vol. i (1878) p. 419. 



3 See, among others, Teall's ' Brit. Petrogr.' 1888, p. 248 ; Harker's ' Petrol, 

 for Students,' 1895, p. 106. 



