140 MESSRS. C. I. GAEDINER & S. H. EEYNOLDS [Feb. 1 898, 



ashy type. Small detached corals occur here and there throughout 

 the rock. 



It appears, therefore, that this conglomerate is not a thrust- 

 conglomerate, but resembles rather the ashy conglomerate of the 

 northern end of the Portraine section. They both have an ashy 

 matrix, and both contain blocks of much the same nature. The 

 limestone-blocks on Lambay Island are more fossiliferous than 

 those at Portraine, and are shown to be of the same age as the Bala 

 Limestone of the mainland. 



The bed is a fairly coarse deposit, and, though many of the blocks 

 are well rounded, one occasionally meets with corals (not attached 

 to any piece of rock) in a comparatively fresh condition, and with 

 angular blocks. This points to the derivation of some of the 

 material from close at hand, but some of the blocks may have been 

 rounded on land and then not moved far from the shore before being 

 finally entombed. 



Summary of the Sedimentary Kocks. 



The sedimentary rocks of Lambay Island are therefore seen to be 

 chiefly of the nature of slates and limestones, and to be contempo- 

 raneous with some of the igneous rocks of the island. On the 

 north-west the slates have yielded no palaeontological evidence of their 

 age, while but little is forthcoming from those on the south-western 

 side. All these slates occur in the neighbourhood of deposits of ashes 

 or tuffs, and may have been laid down during intervals of comparative 

 quiescence, when the neighbouring volcano/was inactive or pouring 

 out its lava in other directions, while the ordinary agents of denu- 

 dation were cutting down its cone and distributing the fragments 

 thereof to a distance. That advancing streams of lava occasionally 

 caught up some of the mud from the sea-floor and baked it appears 

 from the red shales of Carnoon Bay, while the intrusion of coarse 

 porphyry on Heath Hill into the slates shows that igneous activity 

 had not ceased after the deposition of those beds, the baking of the 

 limestone at Kiln Point leading the observer to the same conclusion. 



III. The Igneous Eocks. 



These may be divided for purposes of description into (a) the 

 fragmental rocks, (6) the andesitic rocks, and (c) the coarse por- 

 phyrite. 



{a) The Fragmental Rocks. 



These consist of rocks of every degree of coarseness, from fine 

 ashes to coarse tuffs. There are two localities where they occur over 

 areas of large extent, namely at Scotch Point and along the coast 

 between Talbot's Bay and Carnoon Bay. Between the Castle and 

 Broad Bay there occur several exposures in a tuff-band which runs 

 roughly north and south, while small exposures of ashes associated 

 with the igneous rocks occur here and there throughout the island. 

 Such are the bed between Xnockbane and Calico Hole, and the patch 



