PKECAKBONIPEPvOTJS POCKS OP CHAENWOOD POEE8T. 239 



also ; and he brought the Irish Director-General to Anglesey, where 

 he saw it, and was convinced. 



Mr. Hicks said that he would not now discuss the questions 

 raised in his late paper, alluded to by Prof. Ramsay, but he would 

 willingly take six men into the field, and undertake to prove to 

 them the truth of his views. He eulogized the clear method of the 

 paper just read. The determination of rocks by fossils was easy, 

 but here the age had to be determined by microscopic evidence. 

 He believed that the rocks were not Silurian, but Precambrian ; 

 their general aspect unmistakably told that. 



Mr. Whitakee inquired as to the age of the slate beneath the 

 syenite in the junction represented. 



Prof. Seeley thought that great caution was necessary in accept- 

 ing generalizations which, to him, seemed hardly to be justified by 

 the facts ; as, for instance, from the presence of 70 per cent, of 

 silica to conclude that a rock was a devitrified rhyolite. We must 

 remember that infiltration largely alters rocks, and that laboratory 

 experiments may give fallacious results. In volcanic ashes the 

 broken felspar crystals would become disintegrated, and form clay ; 

 and this would occur in ancient, and not in modern rocks. The 

 broken felspar furnishes uncertain evidence of age. Metamorphic 

 rocks have been much and repeatedly bent, and thus the crystals 

 might be broken. 



Prof. Bonnet, in reply, said that the presence of Precambrian 

 rocks in North Wales appeared to him proved by the examination 

 of specimens, and the evidence of competent observers. His exami- 

 nation of the Wrekin rocks under Mr. Airport's guidance had in- 

 fluenced him much. He could not answer Mr. Whitaker's question 

 further than by saying that the slate appeared to belong to the Porest 

 series. He defended himself against Prof. Seeley's remarks, and 

 maintained that an experienced observer could speak decidedly as to 

 the volcanic origin of rocks with broken crystals of felspar. 



