Vol. 55.] SEEPENTINE AND ASSOCIATED KOCKS IN AN&LESEY. 303 



necessary ■when obvious andesites and basalts, or andesites and 

 trachytes, were found to possess an almost identical chemical com- 

 position, or when good quartz-diorites were found to vary in the 

 amount of silica which they contained to the extent of 22 per cent. 

 The speaker had seen in the field some of the rocks exhibited, and he 

 noticed a resemblance between the Tertiary gabbros of this district and 

 those of Carlingford ; in some Irish localities the Tertiary gabbros 

 were as perfectly mylonized as those in Anglesey, and it seemed 

 possible that the excessive deformation of the more ancient rocks 

 might be due to the repeated earth-movements from which they had 

 suffered. The spherulites in the pyroxenic constituents described 

 by the Authors occupied precisely the same relative position as 

 grains of olivine in the pyroxene of a gabbro from Slieve Foze. 



The Eev. J. F. Blake said that he was glad to be able, on this 

 occasion, to appreciate the value of the Authors' work. From what 

 he had seen of the area he had been convinced that it would repay 

 a more careful examination than he had been able to make. The 

 present contribution was a great step in advance, particularly in 

 the matter of the spherulitic variety, of which the speaker had 

 known nothing. He enquired whether this variety was more 

 abundant near the edge of the mass, so as to suggest that differences 

 of cooling might have caused it ; also whether the dykes or bands 

 of enstatite or diallage might be infiltration-veins ; and he remarked 

 on the sporadic nature of the squeezing of the serpentine, some 

 parts being mylonized and others merely broken. 



Dr. J. W. Gregoey congratulated the Authors on this important 

 contribution to the knowledge of the peridotites of the Irish Sea 

 area. He thought that the Authors' suggestion that the spherulites 

 in the serpentine are not primary structures was probable, from the 

 nature of the spherulites themselves: for the hollow concentric- 

 shelled spherulites, such as that shown on the last of the spherulitic 

 slides, are probably due to secondary changes. Gen. McMahon's 

 suggestion introduced a process which had been advocated by 

 Prof. Stefani, who explained some of the North Italian spherulites 

 as due to accretion and not to spherulitization. 



Mr. Geeenly, Dr. Hicks, and Prof. Seeley also spoke. 



Prof. BoNNEY, in replying, stated to Dr. Gregory that he held the 

 varioles to have been developed in their present position, but not 

 to have always possessed the spherulitic structure ; to Dr. Hicks, 

 that he was well aware that pressures had acted in pre-Cambrian 

 times, but thought that the post-Ordovician pressure had been the 

 cause of the structures mentioned in the paper. As for the occurrence 

 of Pebidian schists, if the St, David's rocks were to be taken as types 

 of Pebidian, then those mentioned in the paper had no claim what- 

 ever (on lithological grounds) to be called Pebidian. To Mr. Blake, 

 he stated that the variolitic serpentine generally occurred near the 

 northern margin of the mass, but that as the radial structure was 

 not supposed to be original, the argument from spherulites would 

 not apply. At the same time, the structure which the Authors 

 supposed to have preceded the spherulites often did occur near the 



