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Those hills stand upon rather a high base, but are all themselves 

 comparatively low. They exhibit each a roundish outline — a character 

 derived from the ready decomposition of the porphyry of which they 

 are composed, and stand in strong contrast with the surface of the 

 country which surrounds them, in which frequently appears the rocky 

 character of a basaltic country. 



In reviewing a country like this, where there are two igneous rocks 

 of different kinds of large extent — a very light- coloured porphyry, and 

 a very dark-coloured trap, which appear not to be contemporaneous — it 

 becomes a matter of interest to determine which of the two is the 

 older rock. That they are not contemporaneous appears from the 

 following comparisons: — 1. The porphyry is of a very light colour, 

 nearly white ; the trap is of a very dark colour, nearly black ; 2. The 

 porphyry is highly crystalline and has a large portion of felspar crystals 

 with some smoke quartz, either as crystals or nodules ; the trap is usually 

 compact and has no quartz crystals ; .3. The porphyry occurs in solid 

 mountain masses. In the two great quarries at Tardree, where stones 

 were got for the long bridge over the Lagan at Belfast, there is a height 

 of face of 50 or 60 feet of it exposed. It has a great uniformity of 

 colour and composition, and has no layers ; the trap in all the sections 

 near Belfast is entirely composed of layers usually differing in character — 

 some hard, some soft ; these layers are often indeed irregular in their 

 thickness, and often thin out to lenticular forms, but still they are layers. 



In the examination of the district, I had hoped somewhere on the 

 exterior boundary of the porphyry to see one or more junctions in which 

 I could see veins of the black trap thrown into the white porphyry, or 

 veins of the porphyry penetrating the surrounding trap, and by this 

 means determine which is the older rock, but I did not see any clear 

 satisfactory junction of the two rocks in contact, nor is there a sign of 

 any such junction round the porphyry district so far as I could discover. 

 I crossed the boundary of the black and the white rocks several times, 

 and saw the surface rock or excavations made in it frequently, but the 

 junctions are obscured by drift or by a considerable depth of the decom- 

 posed sand of one or other of them — sometimes in broad green valleys, 

 sometimes in shoulders of hills or sides of ridges, but nowhere a direct 

 junction of the black rocks ; and, therefore, I cannot say from junctions 

 which is the older and which the newer. 



The tops of the porphyry hills of Carneary and Browndod are much 

 higher points than any of the adjacent trap hills westward or southward, 

 which decline away in elevation towards the River Main, at Randals- 

 town, or the shores of Lough Neagh. On the other hand, Collintop and 

 other basaltic hills on the east, are higher than Browndod or the adja- 

 cent porphyry about Loon Burn. This appears to me as if at that side 

 the trap of Collin were elevated by the porphyry. On the whole, my 

 views lean to the opinion that the porphyry is newer than the trap, and 

 came up through it, the two being now greatly worn down by atmo- 

 spheric action and probable denudation. 



E. I. A. PROC. — VOL. X. 2K 



