304 PROF. A. H. COX AND ME. A. K. WELLS ON THE [vol. lxXVl, 



fig. 4). The progressive deep-seated differentiation had apparently 

 proceeded just far enough to give a felspar-quartz ratio nearest to 

 eutectic proportion. 



Deep-seated differentiation did not, however, cease when this 

 stage of a eutectic magma had been reached. The result of further 

 differentiation was accordingly to produce as a last stage an acid 

 magma, the Crogenen granophyre, which was so highly silicated that 

 the amount of silica exceeded that necessary for a quartz-felspar 

 eutectic. The excess of silica crystallized as quartz, and conse- 

 quently the granophyric structures in the later and most acid rocks 

 are not so perfect as in the earlier rocks of intermediate composi- 

 tion (compare PI. XVIII, figs. 2 & 4). This seems to show that 

 the formation of eutectics does not exert any dominating effect on 

 the process of differentiation. 



(C) Age of: the Igneous Rocks. 



It was already remarked hj Ramsay 1 that the igneous rocks, 

 both acid and basic, were intruded before the main folding move- 

 ments took place. That this was actually the case is at once 

 suggested in the field by the fact that the igneous masses are in- 

 truded approximately along the bedding of the adjacent stratified 

 rocks, and are. therefore, presumably sill-like in character. Further,, 

 detailed mapping shows that the igneous masses are repeated by 

 shatter-faults in almost exactly the same manner as the stratified 

 rocks, while the horizon occupied by any given igneous mass is the 

 same, or nearly so, on each side of the faults, thus confirming 

 the sill-like nature of the intrusions. 



The fact that the intrusions maintain their sill-like character 

 over wide areas shows that they were intruded prior to the main 

 folding ; Avhile the manner in which they are cut and repeated by 

 faults shows that the intrusions also antedated the faulting, 

 both the transverse faulting and the north-east shatter-faulting.. 

 Bat, since the main folding and faulting movements were con- 

 nected with the Caledonian ('post-Silurian') mountain -building, 

 the age of the intrusions is not thus fixed more precisely than as 

 pre-Devonian. 



With regard to the absolute age of the intrusions, the first point 

 to notice is that the granophyres cut through, and are later than, the 

 diabases. Tbe problem limits itself, therefore, to the determination 

 of the age of the granophyres. In this connexion it is highly 

 significant that the granopfyres, while they intrude into the highest 

 volcanic rocks of the district — the Upper Acid Series 2 — never invade 

 rocks above this horizon. On Cader Idris a big granophjTe-sill, 

 petrographically identical with the Crogenen granophyre, cuts into 

 the rhyolites of the Upper Acid Series, but not into the overlying- 

 slates ; while the Crogenen sill itself, on theTyrau-Mawr Gallt-yr- 



1 ' Geology of North Wales ' Mem. Geol. Surv. vol. iii, 1st ed. (1866) p. 29. 



2 A. H. Cos & A. K. Wells, ' The Ordovician Sequence in the Cader-Idris 

 District (Merioneth) ' Rep. Brit. Assoc. (Manchester, 1915) 1916, p. 424. 



