339 



first formed, we may regard the whole viscid mass in the neighbourhood 

 as in a state of equilibrium between the various forces acting on the 

 cooling lava (contraction, &c.) and the pressure of the gaseous contents 

 of the cavity. As cooling proceeds (uniformly suppose) pressure of 

 the latter against the walls of the cavity diminishes rapidly. The 

 various forces are no longer in equilibrium, and the contractile strain 

 will be relieved by the formation of a crack, roughly concentric with 

 the cavity, which, as we might expect, is more regular than it in form. 



The nodules vary in size from that of peas to globes measuring 

 four or five inches in diameter. They have sometimes been flattened 

 by crushing. A soft black substance is occasionally found associated 

 with them. It is sometimes arranged in successive concentric zones. 

 This substance has been analysed by Mr. COLE. Examples of nodular 

 felsites occur in the Pass of Llanberis, near Pont-y-Gromlech ; at the 

 Conway Falls Inn ; at Digoed and at Conway Mountain. 



Porphyritic constituents, as we have already stated, are rarely 

 conspicuous in the Ordovician felsites and felsophyres. Small crystals 

 of both orthoclase and plagioclase may, however, be frequently recognized 

 under the microscope. In some rocks plagioclase alone occurs. This 

 is the case in a nodular felsite from Pont-y-Gromlech. The felsophyre 

 of Arenig is interesting as being conspicuously porphyritic. Macro- 

 scopically it consists of white felspar crystals rarely measuring more 

 than 3 mm. across, soft dark spots evidently representing a ferro- 

 magnesian constituent and a compact greenish-grey matrix. Under 

 the microscope both orthoclase and plagioclase may be recognized. 

 They occur in isolated crystals and in groups. The dark spots are 

 chloritic pseudomorphs after pyroxene. The ground-mass is confusedly 

 crystalline, but it is largely composed of somewhat ragged acicular 

 microlites of felspar which often extinguish at considerable angles. 

 This rock in structure and composition is intermediate between the 

 trachytes and andesites. 



I. II III. IV. V. VI. VII. 



SiO. 72-57 83-302 74-88 79'72 83'08 5O75 08 8 



AU), 13-G4 7-G86 12-00 9-G5 10'25 28'34 14'9 



Fe,().. 2-28 -111 3-50 5-69 tr. 3'G3 -9 



FeO -408 0-20 4'3 



99-47 99-894 100-66 100'53 99-78 99'91 99*4 



I. Purple quartz-fclsite (pro -Cambrian) from Brithdir Farm, near Bangor. Analysis by 

 TEALL. Q.J.G.S., Vol. XXXIX., p. 485. 



II. Felsophyro from Hummit of Aran Mowddwy. Contains porphyritic felspar-crystals in a 



