546 0I^^ THE IGjSTEOTTS EOCKS of the BEEIDDEj!?- TTTT.T. S. 



Old Ked Sandstone period ; but snch rocks are absent from central 

 England. There is one area which may afford some cine to this 

 question, and that is the N'.W. corner of the Corndon district, where 

 similar diabases break through Cambrian and Silurian rocks. I 

 have already begun to work there with the object of determining 

 this point amongst others, but have not yet arrived at any suffi- 

 ciently definite results. 



Disctrssiox. 



Prof. T. Etjpeet Jones, while thanking the author of the paper, 

 suggested that he should refer to the several groups of strata 

 rather than use the names Cambrian and Silurian, the appli- 

 cation of which is uncertain. Avoiding that subject, he wished to 

 ask about the nodular trap-rock on the Shrewsbury road at the foot 

 of the Breidden. 



Mr. Cole had examined the Corndon masses and found a striking 

 similarity between them and the rocks described by the author. 

 He had himself found enstatite in the Corndon rocks, among 

 which were probably some andesitic glasses. 



Mr. Teall congratulated the author on his valuable paper. He 

 would like to see some agreement as to the use of the terms ande- 

 site and porphyrite. Most of the so-called porphyrites that he had 

 examined were merely altered andesites, and he was at present in- 

 clined to use the term in this sense. If this view were adopted, then 

 the interbedded igneous rocks described by the author would have 

 to be called porphyrites. This, however, was a very trivial 

 criticism. The intrusive enstatite diabases were probably very 

 similar in chemical composition to the altered andesites. 



The AuTHOE said that he had endeavoured to avoid the difficulties 

 about understanding the sense in which " Cambrian " and " Silurian" 

 were employed, by using local names for the several rocks. He 

 thought the term porphyrite might be restricted to certain classes of 

 altered andesites. 



