Vol. 54.] ON THE BALA & OTHER ROCKS OF L1MBA.Y. 147 



The coarse porphyrite, therefore, usually occurs in dykes or sills, 

 but it is also found as an extruded mass on Flint Hill and Pilot's 

 Hill. 



With regard to its mineralogical constitution, we have nothing to 

 add to Von Lasaulx's account. We have recognized porphyritic 

 felspars allied to labradorite, much decomposed, in a groundmass 

 ■of lath-sha]ped felspars and augite-granules. Calcite, epidote, mag- 

 netite, pyrites, and sphene also occur. 



IV. Conclusions. 



From the observations recorded herein, it seems plain that in 

 Bala times Lambay Island was close to a centre of vulcanicity. 

 A vent could not have been very far distant, and its j9[anks must 

 have extended close to the districts of Lambay and Portraine. 



The main extrusions of lava were of a very normal type of andesite, 

 augite- and hypersthene-andesites being, however, represented to a 

 small extent on the island, while the coarse Lambay porphyrite 

 was injected into these andesites, and must occasionally have 

 reached the surface. That some of the lavas flowed beneath the 

 sea seems probable from the occurrence among them of beds of 

 slate, while that the period of igneous activity had not ceased 

 when the sedimentar}^ rocks of Heath Hill had accumulated is 

 shown by the intrusion of the coarse porphyrite into the slates of 

 that hill, and the baking of the limestone at its eastern edge. 



The palseontological evidence is scanty, but, such as it is, it points 

 to the sedimentary rocks being of the same age as the Portraine 

 beds, namely, Upper or Middle Bala. 



Hence, in Bala times, there was an immense ontpouring of 

 andesitic rocks in this district, while explosions shot out fragments 

 which formed ash-beds at various places, and cracks in the andesites 

 were filled with a coarsely porphyritic rock, which also here and there 

 welled up to the surface. Meanwhile limestones and shales were 

 "being deposited round the volcano, and were in part subjected to 

 the ordinary agents of denudation. Fragments of the igneous rocks 

 -and of these upraised sedimentaries were rolled down to the shore 

 and piled up to form a massive conglomerate, which, as it sank 

 beneath the waves of the Bala sea, was covered by calcareous mud ; 

 and therewith the history of Lambay Island in Bala times is brought 

 to a close. 



In conclusion we would offer our best thanks to Mr. W. W. Watts, 

 for the assistance which he has so kindly given us ; to Mr. F. R. Cowper 

 Reed, for naming our fossils ; and to the authorities of the Geological 

 Museum in Dublin, for the generosity with which they have afforded 

 us every facility for examining the collections of rocks and fossils in 

 their possession. 



