part 1] IGNEOUS AND ASSOCIATED BOCKS OE LLANWRTYD. 33 



(7) Conclusion. 



When this investigation was nearly complete, we exhibited a 

 series of specimens from Llanwrtyd at the conversazione of the 

 Geologists' Association in November 1920. We learnt then that 

 Mr. W. Le Lacheur, F.G.S., had for several years been interested 

 in the district, and had examined petrographically most of the 

 rock-t} r pes. Mr. Le Lacheur had, therefore, a prior claim to 

 publish the results of his work, and we wish to record our sincere 

 appreciation of his great kindness in allowing us to publish this 

 paper. Not only has he allowed us access to his collection of 

 rocks and slides, but he has also placed at our disposal the results 

 of his own observations in the field. 



We also wish to express our thanks to Miss G. L. Elles for the 

 determination of the graptolites ; to Prof. A. H. Cox for valu- 

 able suggestions and for reading part of the manuscript ; and to 

 Mr. A. K. Wells for reading the manuscript, and for helpful 

 criticism. 



Part II. Petrographical. (By S. W. W.) 



(1) Introduction. 



It may be stated at the outset that the dominant feature of the 

 igneous rocks which are here described is the almost universal 

 occurrence of albite. All the rocks, with the exception of the 

 Pen-y-banc intrusion, belong to the spilite-keratophyre series as 

 defined by Mr. H. Dewey & Dr. J. S. Flett.l The lavas are 

 essentially albite-bearing rocks, the Upper Ashes resemble quartz- 

 keratophyres in composition, while the other pyroclastic rocks of 

 the series have, without exception, spilitic affinities. 



Prof . A. H. Cox 3 has noted the predominance of pyroclastic rocks 

 among those of spilitic composition, and the Llanwrtyd district 

 affords yet another example of this feature, which may be cor- 

 related with high viscosity of the magma. 



The occurrence of spilitic rocks at the horizon defined above 

 (p. 31) is interesting; but remarks on this matter may be deferred 

 until the petrography of the rocks has been outlined. 



The descriptions follow the stratigraphical order of the rocks. 



(2) Tiie Lower Ashes and Breccias. 



The features presented by these rocks in the field have already 

 been noted (p. 18). Microscopical examination of the typical rock 

 shows that it consists of fragments, originally glassy, set in a 

 silicified matrix which presents a flinty or felsitic appearance. 



1 Geol. Mag. 1911, pp. 202, 241. 



2 Eep. Brit. Assoc. (Birmingham, 1913) p. 497. 



Q. J. G. S. No. 313. d 



