290 PROF. A. II. COX AND MB. A. K. WELLS Ofr THE [vol. lxxvi.. 



(ii) The smaller Tyn-y-llwyn intrusion is composed, as regards 

 its lower portion, of a fine-grained truly basic rock ; upwards it. 

 becomes coarser in grain and somewhat less basic in composition — 

 Tyn-y-llwyn type — but still remains more basic than the mark- 

 fieldite type. The coarser and less basic rock contains numerous 

 xenoliths of the tine-grained basic type. 



(iii) The larger Tyn-y-llwyn intrusion contains a number of 

 types showing every transition from basic to acid rocks. The 

 rocks along its lower margin were evidently formed almost simul- 

 taneously with those in the smaller Tyn-y-llwyn intrusion below, 

 the same types being present in both intrusions. Upwards, however, 

 the highly - basic granophyres and rocks of Tyn-y-llwyn type 

 gradually become less basic, and pass into the coarse-grained inark- 

 tieldite type. This in turn passes into a finer-grained and truly 

 acid rock which cannot be distinguished from the typical Crogenen 

 granophyre. The coarse markfieldite type and the acid Crogenen 

 or eurite type are connected by a transitional zone in which the 

 rock, while it cannot be distinguished from the eurite type along 

 surfaces showing a fresh fracture, j^et weathers grey instead of 

 white or vermilion. The uppermost zone of this intrusion shows 

 in one locality a return to the somewhat basic markfieldite type, 

 chilled at the actual contact. 



(iv) The Waen - Fechan intrusion, though stratigraphically 

 higher than the other minor intrusions, yet consists of a thoroughly 

 basic rock (Waen -Fechan type) comparable with, but even 

 more basic than, the lower half of the Craig-y-Castell mass. 



(b) Petrology of the granophyric rocks. (1) The 

 Crogenen granophyre. (r<) The normal acid rocks. — 

 Macroscopic characters : The granophyre is exceedingly uniform 

 over the whole extent of its outcrop as regards its appearance in 

 hand-specimens, always excepting the marginal modifications that 

 will be described later (p. 295). The fresh rock is grey-blue, 

 and presents a fine-grained ' felspathic ' appearance, no distinct 

 minerals being identifiable, apart from small and scattered felspar- 

 phenocrysts. These are lath-shaped in section, and show a vitreous- 

 lustre. They persist even into the basic modifications of the 

 granophyre, and form a ready means of distinguishing such modifi- 

 cations from the true diabases. 



The normal rock weathers white, while joint-surfaces are often 

 blotched irregularly with yellow or occasionally with vermilion. 

 Just beneath the weathered crust little rounded white spots may 

 frequently be observed ; these spots represent micrographic quartz- 

 felspar aggregates, but they are not visible in the fresh specimen. 

 The rock breaks with a smooth fracture, yielding chips with 

 knife-like edges comparable to those derived from rhyolites. The 

 gravelly sub-soil material derived from the granophyre shows, 

 however, a more reddish tinge than the corresponding material 

 from a rhyolite. 



