﻿Vol. 6 1.] ARENIG EAWR AND MOEL ELYFNANT. 629 



Granite/ In composition they agree exactly with the Lower Ashes 

 and Agglomerates, as also with many of the lavas and sills of the 

 Stapely Volcanic Series of West Shropshire, but are more thoroughly 

 silicified. In general, the intrusions take the form of rather thick 

 sills, and occur above or below the Great Agglomerate. They are 

 also frequent among the Platy Ashes above the latter, and, as 

 shown in the map (PI. XLI), extend downwards almost to the 

 Llyfnant or Ecctensus-Flags, and upwards as far as the Daerfawr 

 Shales. 



In several places, as at Bryniau Poethion or about Llyn Crafanc, 

 actual transgressions can be made out. The thickness of the sills 

 varies enormously, but even the centre of the largest sill which is 

 quarried at Clogwyn-y-Fran close by Arenig Railway-station, and 

 is quite 200 feet thick, has spasmodic large vesicles ; and smaller 

 sills, which have lost the filling of their amygdules, frequently 

 appear quite spongy. All the sills are more or less columnar in 

 their jointing, the thicker almost as much so as the basalts of the 

 Giants' Causeway. The best columns seem always to belong to the 

 under, and not to the upper, surface of the sills. 



The question of the intrusive or extrusive origin of the sills has 

 been an open one ever since the time of Sir Andrew Ramsay, who 

 was unable to make up his mind on the subject, and it is only after 

 a good deal of work that I have been able to conclude that they 

 are in the main intrusive. Even now, I picture some submarine 

 volcanic cone sealed between successive eruptions by the chilling 

 action of the sea, until eventually it became easier for the welling 

 lava to spread itself among the accumulated rubbish than to burst 

 asunder the cone and pour out over the surface. There is no 

 corresponding set of intrusions among the ashes of the Upper Series, 

 which have suffered far more contemporaneous decomposition than 

 the members of the Lower. For this, and for other reasons, I am 

 inclined to regard the Upper Series as, in part at least, a series of 

 subaerial accumulation. 



The total mass of the andesite-intrusions must be enormous, 

 and is quite as great as the whole of the rest of the Lower Volcanic 

 Series. Like the Great Agglomerate, the intrusive sills are always 

 thicker and more abundant in the north-east than in the south- 

 west of the district, 



Under the microscope, it is seen that the rocks were formerly 

 andesites, but that they have undergone a great deal of subsequent 

 silification and other alteration. The sole really-fresh original 

 minerals are the felspars and the apatite. The former are beauti- 

 fully preserved, and are mostly oligoclase or andesine, usually with 

 narrow extinctions and showing Baveno or window-twinning, in 

 addition to the ordinary albite-twinning. Sometimes the crystals 

 are beautifully formed, while in others the growth has been checked 

 at the skeleton or spongy stage. A certain amount of orthoclase 

 is occasionally met with, but is not especially abundant. The 

 apatite is extremely characteristic, and occurs in elongate hexagonal 

 needles which are unusually large for so fine-grained a rock. 



