250 MESSES. T. C. CANTRILL AND H. H. THOMAS ON THE [May 1906, 



the same petrographical province. The order of extrusion was : 

 (1) augite-andesites ; (2) rhyolites; (3) augite-andesites, with some 

 hornblende-andesite ; and (4) intrusions of diabase and porphyry. 



We adopt the view that the extrusive rocks are associated with 

 the lower members of the Tetragraptus-Beds, and are consequently 

 of Lower Arenig age ; while the intrusive rocks have been injected 

 into the extrusive rocks, and have also affected the Tetragraptus- 

 Beds. 



EXPLANATION OF PLATES XXIII-XXVI. 

 Plate XXIII. 



Geological map of the igneous and associated rocks of the Llangynog 

 district, on the scale of 3 inches to the mile. 



Plate XXIV. 



Fig. 1. Devitrified spherulitic rhyolite [E 4143] ; crossed nicols. Some of the 

 spherulites show a rude blaek cross. X 17. See p. 241. 

 2. Partly-devitrified perlite [E 4443] ; ordinary light. X 17. See p. 242. 



Plate XXV. 



Fig. 1. Vesicular augitic andesite [E 4138] ; ordinary light. X 17. The 

 figure shows lath- shaped felspars, set in a groundmass that exhibits a 

 structure approaching the variolitic. At the top of the field is a small 

 vesicle filled with chlorite, and at the bottom a pseudomorph after 

 augite in chlorite and secondary quartz. The chlorite shows slightly 

 darker than the quartz. See p. 239. 

 2. Pilotaxitic andesite [E 4439] ; ordinary light. The figure shows a 

 mass of felspar-microliths, and a solitary felspar of an earlier 

 generation at the top of the field, x 30. See p. 240. 



Plate XXVI. 



Fig. 1. Andesitic tuff [E 4163] ; ordinary light. The figure shows several 

 angular fragments of hyalopilitic andesite, with minute microliths 

 arranged with flow-structure. At the bottom of the field is a larger 

 fragment of andesitic pumice, the vesicles of which are filled with 

 chlorite. The larger fragments are set in a fine groundmass of 

 broken felspar-crystals and microliths. X 17. See p. 241. 

 2. Ophitic diabase [E 4131] ; crossed nicols. Shows ophitic augite and 

 partly-decomposed felspars. X 17. See p. 244. 



Discussion. 



The President said that he was glad to find that the Authors 

 had obtained definite evidence of two distinct sets of movements — 

 the one before, the other after, the deposition of the Old Eed Sand- 

 stone. He himself, when working with the late Mr. Thomas Roberts 

 in the Haverfordwest district, had been unable to obtain evidence 

 bearing on this question, owing to the profound faulting which had 

 there occurred. 



Prof. Groom said that geologists would heartily welcome another 

 incursion into the obscure geology of the Welsh borderland. 

 Knowing the ground himself, he was convinced of the accuracy 



