﻿Vol. 6r.] BETWEEN ST. DAVID'S HEAD AND STRUMBLE HEAD. 603- 



enstatite into chlorite, and examples occur in which the chlorite- 

 patches contain nuclei of enstatite. The chloritic substance in the 

 lime-bostonites has already been mentioned. 



Less frequently are found brightly-polarizing, radially-aggregated 

 zeolitic species. Sericite and opaque amorphous 

 granular matter are common alteration-products of the felspars 

 in the southern area. I noticed also that the Llanwnda type 

 develops asbestiform shear-planes locally, a phenomenon 

 which I never observed in the rocks between St. Nicholas and 

 St. David's Head. 



VII. Mutual Relations of the Magmas. 



I have endeavoured to show that there is evidence in this area 

 of three distinct types of intrusion, namely: (1) the gabbro- 

 type, including the diabase of Fishguard and Prescelly, which I have 

 designated the Llanwnda type ; (2) the norite and enstatite- 

 diorite type of St. David's Head; and (3) the lime-bostonite 

 type of the Abercastle- Mathry district. We may consider 

 what evidence there is as to the age of these intrusions, and 

 whether they may be regarded as the result of differentiation from 

 an original magma or the product of distinct intrusions from 

 different deep-seated sources. Prof. Brogger's explanation of the 

 origin of the bostonites and camptonites of Gran, 1 which he 

 regards as complementary dykes resulting from the differentiation 

 of an olivine-gabbro- diabase magma, can scarcely be applied to the 

 present case, in the absence of a larger series of chemical analyses. 

 In the case of the lime-bostonite series of Abercastle, however, the 

 evidence seems to point to the conclusion that these are of earlier 

 date and quite distinct from either of the other types. In the 

 first place, they were evidently intruded before the main faulting 

 of the district took place, since these narrow sills are abruptly 

 truncated by the Pwll-Strodyr Pault. Moreover, they are sharply 

 distinguished from any other intrusions in this area, and do not 

 show, so far as I have observed, any gradual passage into, or 

 admixture with, either the quartz-norites on the one hand, 

 or the diabase-group on the other. Mr. Cowper Reed brings 

 forward evidence to show that the Waterford intrusions are later 

 than the first post-Ordovician folding and earlier than the pre- 

 Upper Old-Red-Sandstone denudation. 2 Messrs. Kilroe & McHenry 

 assign the intrusive tuff-like rocks of the south-eastern corner of 

 Ireland to the Old-Red-Sandstone age. 3 The evidence for these 

 conclusions seems to be fairly complete. I have no acquaintance 

 with the Waterford district personally, but the lime-bostonites and 

 porphyrites of Abercastle so strongly resemble some of these Irish 



1 ' Basic Eruptive Eocks of Gran ' Quart. Journ. Geol. Soc. vol. 1 (1894) p. 26. 



2 'Igneous Eocks of the Coast of County Waterford' Quart. Journ. Geol. 

 Soc. vol. lvi (1900) p. 690. 



3 ' Intrusive, Tuff-like, Igneous Eocks & Breccias in Ireland ' Quart. Journ. 

 Geol. Soc. vol. lvii (1901) p. 488. 



