252 IGNEOUS AND SEDIMENTAKY EOCKS OF LLANGYNOG. [May 1906, 



the great break at their base, and from the fact that they consist 

 of marls, sandstones, and cornstones, indistinguishable except in 

 colour from the lower part of the Old Red Sandstone ; moreover, 

 they pass by alternation upwards into the ordinary red marls. 

 Finally, they are unfossiliferous, and, until they yield a Silurian 

 fauna, they can be regarded only as the basement-beds of the. Old 

 Red Sandstone. 



In reply to Mr. Fearnsides, he considered that the Tetragraptus- 

 Beds should be taken to include the zones of Didymograptus hirundo 

 and D. extensus ; but in the Llangynog district the lower beds cease 

 to yield graptolites, and therefore may belong to the B-exUnsus 

 Zone or to a lower horizon of the Arenig. 



Mr. Thomas, after thanking the President and Fellows for their 

 kind reception of the paper, said that he was in perfect agreement 

 with the remarks made by Mr. Cantrill. In reply to Mr. Elsden, 

 he said that it was quite possible that the two types of intrusive 

 rocks (diabase and porphyry) might be the dyke-equivalents of the 

 andesite and the rhyolite respectively ; with regard to the occurrence 

 of rhombic pyroxenes in the diabases or andesites, careful search 

 had failed to reveal any pseudomorphs after these minerals. 



