24 LOTVER CAEBONiEEEors [vol. Ixxiii, 



other formations. The problem of albitization had attracted con- 

 siderable attention from British geologists since Mr. G-rabham 

 noticed albitization among the Scottish Carboniferous lavas. A 

 notable recent contribution was that of Mr. McLintock, who had 

 studied the Tertiary propylites — to use the late Prof. Judd's 

 nomenclature — of the island of Mull. It was best to discuss the 

 origin of albitization in every case upon the local evidence. In 

 connexion with the present paper, which added considembly to our 

 knowledge of the Derbyshire volcanic rocks, it w^as important to 

 know whether the accompanying analyses could be fully relied 

 "upon. 



Dr. A. H. Cox wished to congratulate the Author on the results 

 of the investigations. He had been pai'ticularly interested in 

 hearing these results, since they confirmed opinions at which he had 

 previously amved from theoretical considerations. The President 

 had already referred to the Derbyshire rocks as being intermediate 

 in chai'acter between the Lower Carboniferous lavas of Devon 

 and Cornwall on the one side, and those of Scotland on the other. 

 The stratigraphical conditions were also intermediate. The lavas 

 of the West of England were extruded in an area which, having 

 undergone prolonged subsidence, w^as geosynclinal in character, 

 and the rocks were typically spilitic. On the other hand, the 

 Lower Carboniferous of Scotland was a shallow-water formation, 

 implying only a limited amount of subsidence, and the lavas were 

 typically basaltic, sometimes, it was true, showing a certain amount 

 of albitization. Opposed to this was the Lower Carboniferous of 

 Derbyshire — a massive limestone formation, evidently deposited in 

 deeper water, implying a greater degree of subsidence, though not by 

 any means to such a degree as that which had obtained in the West 

 of England. It was to be expected, therefore, that the Derbyshire 

 lavas, while very largely basaltic, would yet show a closer approach 

 to the spilitic type than was manifest in the case of the Scottish 

 rocks. The whole question of a possible connexion between the 

 iype of earth-movement and the composition of the igneous rocks 

 produced during such movement was still an open one, and papers 

 such as the Author's were most valuable as furnishing the data by 

 means of which this important subject could be investigated. 



The Author, in a brief reply, desired in the first place to thank 

 the President and Fellows for their kind reception of his paper. 



In regard to Mr. Bailey's view that it was misleading to call an 

 olivine-bearing rock a spilite, he pointed out that, since the spilites 

 appear to be derived from the basalts, and since there are both 

 olivine-basalts and basalts without olivine, there seemed to be no 

 valid reason why there should not also be olivine-spilites and 

 spilites without olivine. 



It had been a question of some difficulty whether the Derbyshire 

 rocks should be placed with the spilites or with the mugearites : 

 he thought, however, that, in view not only of their structure but 

 also of their physical environment, they came nearer the former 



