part 2] GEOLOGY OF THE TREFItlW PYRITES DEPOSIT. 115 



Trefriw deposit would appear to be of the latter type. It was 

 exceptional, however, in that it was connected with diabases, and 

 not with porphyries, as was the case in County Wicklow and in the 

 Rio Tin to region. According to the Author, it had been formed 

 by the contact-metasomatism of a local development of the well- 

 known pisolitic iron-ore of Bala age. There was much to be said 

 for this view, but two other alternative hypotheses might be con- 

 sidered : first, that the deposit represented the more usual replace- 

 ment of slate along a graptolitic horizon, by waters emanating from 

 an acid intrusion ; or, secondly, that it had been formed by the 

 pyritization of the pisolitic iron-ore, by the action of sulphuretted 

 hydrogen of organic origin, immediately after deposition, as had 

 happened to the uppermost layers of the main seam of the Cleve- 

 land ironstone of Yorkshire. 



The Author thanked the Fellows for their kind reception of 

 his paper. In reply to Dr. Cox, he said that the beds containing 

 Ample xocfr apt us arctus were regarded as part of the Nemagra'ptus- 

 gracilis Zone. 



With regard to the points raised by Prof. Cullis, the pyrites 

 seemed to be intimately associated with the diabase, covering its 

 undulating surface in a uniform sheet. Also the upper part of the 

 diabase contained a considerable amount of pyrites, which, although 

 it might be derived from an older pyrites-mass, was more probably 

 formed at the same time, so that the ore-body was later than 

 the diabase and not formed in association with previous acidic 

 intrusions. 



Again, it was said that if it be admitted that pisolitic ironstone 

 may have been altered into pyrites, this might have occurred on 

 the sea-floor, in Bala times, almost contemporaneously, and have 

 nothing to do with the diabase. The p}Titic ore, however, occurs 

 only in contact with the diabase, although pisolitic iron-ore and 

 black graptolitic shales are found in many places in Anglesey, 

 Carnarvonshire, and Merionethshire, occurrences which again con- 

 firm the association of the pyritic ore and the diabase. 



Q. J. G. S. No. 294. 



