374 H. HICKS ON THE OCCURRENCE OF 



ordinary processes, being fixed even in the fire, it seems probable 

 that we must look for some other cause in addition to the heat, 

 which would be capable of effecting this change. The extraordinary, 

 heat of so large a mass of melted matter, combined with the enormous 

 pressure exerted at the same time upon the rock, may, however, 

 have been capable alone of producing changes of which we have but 

 little conception. Still I think it is probable that the change chiefly 

 took place through the influence of watery vapour and gases generated 

 at the time in the rock itself, or communicated to it from the in- 

 trusive vein. 



The bedded rock, at the time this intrusion into it took place, was 

 in the horizontal position, and doubtless in a plastic state and with 

 many thousands of feet of other deposits superimposed upon it. These 

 intrusive masses appear almost to follow the curves of the strata and 

 to run in the direction of the line of the bedding along the coast ; 

 but they gradually get higher in the series in an easterly direction, 

 and the direction of the flow seems evidently to have been from 

 south-west to north-east. 



Whenever the sedimentary beds come in contact with these 

 trap dikes they always show an altered state ; and as these masses can 

 be traced for several miles, their effect on almost every bed of the 

 Menevian group may be seen ; for though the general direction is 

 that of the line of bedding, yet in their course they gradually ascend 

 and ultimately cross the series. The invariable effects produced are 

 a deepening of the colour of the rocks, cleavage, loss of lime, of 

 phosphoric acid, and of every evidence of such substances (as the 

 horny texture of the shell of the Brachiopoda, the Crustacean shell, 

 &c), but with the fossil impressions left, in a compressed and 

 distorted state. In addition to these changes, thin lines of iron 

 pyrites are frequently seen running in the direction of the bedding 

 in those beds which have undergone any considerable alteration in 

 their character ; and as, according to Mr. Hudleston's analysis, 

 these altered beds are now deficient in pyrites, it seems evident that 

 this must have been derived from them. 



The analyses which I have made of portions taken from the 

 intrusive dyke in the section at Porth-y-rhaw gave the following 

 results : — (1) Under surface resting on black slate, P 2 O g 0*60 per 

 cent, carbonate of lime 15 per cent ; (2) Centre of mass, P 2 5 0*40 

 per cent, carbonate of lime in small quantity only ; (3) Upper sur- 

 face of second mass, P 2 5 0-50 per cent, carbonate of lime 8 per cent. 

 Between the masses of trap there are, in black altered slate, veins of 

 calcareous spar in a very pure state, and which when analyzed 

 scarcely gave a trace of P 2 5 . From these analyses it will be seen 

 that phosphoric acid is distributed throughout the whole mass of 

 the trap, but that it and the carbonate of lime occur in very 

 great excess towards the surfaces. 



If these dykes in forcing their way through the beds did not take 

 up some of the ingredients which they were directly or indirectly 

 the means of setting free from these beds, it is but natural to sup- 

 pose that they must have frequently, indeed usually, have had their 



