part 1] IGNEOUS AND ASSOCIATED BOCKS OE LLANWKTYD. 4$ 



described as an albitized olivine-basalt. He also compared 

 it with certain dykes in the pre- Cambrian rocks of Pembrokeshire- 

 described by him. 1 



The age of the intrusion is unknown. Evidence has been 

 brought forward (p. 24) which indicates that it preceded the 

 formation of the western boundary-fault. This fault is later than' 

 the folding, since it truncates the northern end of the anticline. 

 The latter structure is approximately Caledonian in trend. Farther 

 than this it is not possible to go. 



(7) Summary and Conclusions. 



The igneous rocks exposed at Llanwrtyd "Wells are predominantly 

 pyroclastic in nature, while their composition places them in the 

 spilite-keratophyre series almost without exception. There is a 

 noteworthy absence of intrusive rocks. 



The succession begins with 'rhyolitic' breccias the exact nature 

 and composition of which are rather in doubt. While not ex- 

 cluding the possibility of their being flow-breccias, we have taken 

 the available evidence as indicating a probable agglomeratic 

 nature. 



The spilite-breccias which occur at the base of the flows are 

 certainly of explosive origin. They pass gradually upwards into- 

 spilites which are true albite-bearing rocks. A normal pillowy 

 facies can be distinguished from a massive non-pillowy type. 



The spilites are succeeded by a considerable thickness of 

 sediments, in which occur bands of halleflinta and of crystal-tuff ; 

 while at the top of the series is a good development of quartz- 

 keratophyre tuffs. 



The close resemblance between some of the pyroclastic types- 

 described, and those of other areas differing widely in horizon, tends- 

 to emphasize the unity and genetic individuality of the spilite- 

 keratophyre suite. 



The Pen-y-banc intrusion is a somewhat peculiar rock. It has 

 been possible to compare it with the marleosites of Skomer Island 

 and with certain pre- Cambrian dykes in Pembrokeshire. Its age 

 is unknown, and it falls apart from the rest of the series, its only 

 point of affinity with these rocks being the presence of albite. 



It may be remarked, in conclusion, that it is interesting to find 

 spilitic rocks so high in the Ordovician sequence. Dr. Harker has 

 indicated in his recent address to the Geological Society 2 that 

 Llandeilo times witnessed a change in the stress conditions, which 

 effected a general replacement of spilites by andesites. He notes 

 instances, however, of the persistence of the spilitic type, as on 

 the summit of Cader Idris. The Llanwrtyd district affords another 

 instance of this persistence ; the sequence here is closely parallel 



1 H. H. Thomas & 0. T. Jones, Q. J. G. S. vol. lxviii (1912) pp. 389-90. 



2 Q. J. G.S. vol. lxxiii (1917-18) p. lxxvi. 



