﻿632 ME. W..G. FEARNSIDES ON THE GEOLOGY OP [Aug. 1905, 



The granular andesites occur in a series of rather small 

 sills, which seem to be arranged en echelon along the bedding- 

 planes among the JS r iobe-Beda. They contain just a few rounded 

 phenocrysts of oligoclase or orthoclase, but are, on the whole, non- 

 porphyritic. They closely resemble the groundmass of andesites of 

 the main series, but are even more thoroughly recrystallized, and 

 show quite a coarse-grained mosaic of micropoecilitic felspar. The 

 original microlites are short and rather stumpy, and appear to show 

 good flow-structure. The microlites are arranged without any 

 reference to the micropoecilitic patches, and frequently lie across 

 the boundary between two of them (4666-68). On weathering, 

 a great deal of calcite is formed between the micropoecilitic patches, 

 and this, dissolving away upon the surface, gives the rock a very 

 characteristic sugary appearance in the field. 



The hornblende-porphyrites are only represented in this 

 district by a single sill, but as this is exposed upon its dip-slope, it 

 occupies a very considerable tract of ground, which unfortunately, 

 however, is mostly covered with Drift. Where seen, the rock seems 

 very like the more basic porphyrites of E-hobell Fawr, 1 and contains 

 delightful porphyritic crystals of green hornblende and well-formed 

 Carlsbad twins of a plagioclase-felspar (4669-71). Unfortunately, 

 no specimens fresh enough to slice are as yet to hand, and a further 

 consideration of this rock must therefore be postponed. 



The Cleavage. 



Considering its geological position in the midst of the greatest 

 slate-producing region of the world, the district of Arenig Fawr and 

 JVloel Llyfnant is remarkably free from cleavage. When one 

 follows the outcrop of the various series to the south or to the north- 

 west for even only a few miles, one is struck by the notable increase 

 in the intensity of the cleavage ; and hence, though not able to bring 

 forward much evidence, I am inclined to think that the absence of 

 cleavage here is due to the accident of the district being, as it were, 

 the keystone of the great Harlech dome. 



Further, I find that the cleavage of North Wales, as a whole, has 

 a Caledonian or north-easterly to south-westerly trend, and that the 

 average strike of the rocks of the Arenig area is transverse to this. 

 I should therefore suppose that the great masses of unyielding and 

 uncleaved igneous rocks of the district may have transmitted earth- 

 pressures along their length without suffering much deformation, 

 and so may, for a time at least, have held off the stresses from the 

 adjoining softer rocks. I notice also that there occur in the area no 

 rocks the trend of which is now exactly north-west to south-east, 

 but that all strikes change abruptly at a series of faults from north- 

 and-south almost to east-and-west. Such faults I regard as marking 

 the stage when the rigid igneous rocks could no longer stand the 

 strain, and so gave way, as it were, with a rush ; and I should like 



1 G. A. J. Cole & T. H. Holland, Geol. Mag. 1890, p. 452. 



