44 DR. STAMP AND MR. WOOLDRIDGE ON THE [vol. lxxix, 



to that on Cader Idris, as appears from the correlation-table 

 (p. 32). 



I wish to express my very great indebtedness to Mr. A. K. 

 Wells for much help and encouragement during my work. I have 

 further to thank him for the photomicrographs which illustrate 

 this paper. My thanks are also due to Prof. W. T. Gordon for 

 critically reading the manuscript, and to Dr. H. H. Thomas 

 for examining slides of the intrusion and advising me as to its 

 affinities. I also desire to associate myself with my co-author in 

 the thanks expressed to Mr. W. J. Le Lacheur, to whom I am 

 especially indebted for the loan of microscope-slides. 



EXPLANATION OP PLATES I & II. 



Plate I. 



Fig. 1. ' Bhyolitic ' breccia, showing passage in one fragment from a perlitic 

 glass to a fine-grained spilite : X 20. (See p. 34.) 



2. Spilite-breccia, showing in the lower part of the field a highly vesi- 



cular spilite-fragment. Well-developed bogen-struktur is seen 

 above : X 20. (See p. 35.) 



3. Spilite, showing vesicles surrounded by dark spilitic rims, with tan- 



gentially arranged felspar-crystals : X 20. (See p. 37.) 



4. Vesicle in spilite. The wall has been ruptured, admitting the fluid 



magma, and secondary albite is seen outlining the original vesicle- 

 wall : X 20. (See p. 38.) 



5. Quartz-keratophyre tuff from the Upper Ashes. Embayed quartz- 



fragments are seen, with a felspar-crystal in the upper part of the 

 field : X 20. (See p. 40.) 



6. Intrusion (albitized olivine-basalt). A well-formed bastite-pseudo- 



morph after olivine in the centre of the field, with others in the 

 lower part. Several large plates of ilmenite are seen enclosing the 

 ends of felspar-laths subophitically. Fluxion-structure is also well 

 seen : X 20. (See p. 42.) 



Plate II. 



Geological map of the neighbourhood of Llanwrtyd Wells, on the scale of 

 3 inches to the mile, or 1 : 21,120. 



Discussion. 



Prof. A. H. Cox remarked on the great interest of the paper to 

 him, since it dealt with an area so similar to, and yet so distant 

 from, the Cader Idris district which he himself was examining. 

 He had had the advantage, through the Authors' courtesy, of 

 reading a part of the paper, and of examining many of their spe- 

 cimens. The correspondence between the rock-t} r pes of the two 

 districts is truly remarkable, and all the more so since it was quite 

 unexpected when the investigation was originally commenced. It 

 was then naturally assumed that the Llanwrtyd rocks would corre- 

 late with those in the adjoining Builth area. The proof now 



