108 PEOOEBDINGS OF THE GEOLOGICAL SOCIETY, [Jan. 11, 



stratified between perfectly unmetamorphosed beds of sandstone and 

 limestone, without any alteration at the points of contact, such as 

 would be produced by an igneous rock. He also cited the crumpled 

 strata in the Maritime Alps, in which the granites were parallel with 

 the other beds, and seemed to form part of them. 



Mr. Cakruthers mentioned that the late Prof. Fleming, twenty 

 years ago, had taught the same doctrine as to the nature of granite 

 as that held by the last speakers. He also stated that similar views 

 would be found expressed in Headrick's ' Mineralogy of Arran,' 



Mr. David Porbes agreed that the crumpling of the strata was 

 not due to the intrusion of any eruptive rock. He completely dis- 

 agreed with Prof. Eamsay and the author as to the origin of granite, 

 and maintained that, in the sedimentary rocks traversed by the gra- 

 nite, the requisite ingredients for the formation of granite did not 

 exist. The proportion of felspar in quartzose rocks was infinitesimally 

 small, as compared with that entering into the composition of granite. 

 He could not accept the notion of the heat from the interior approach- 

 ing gradually to some portion of the surface. 



Prof. Eamsat, in reply to Mr. Forbes, maintained that some of the 

 slaty rocks of Wales, by extreme metamorphism, would pass into 

 some kinds of granite. As to the conditions of metamorphism of the 

 rocks, this process must have gone on at a time when these older 

 rocks were overlain by a great thickness of more recent beds which 

 have since been removed by denudation. 



2. On the connexion of Volcanic Action witJi Changes of Level. 

 By Joseph John Murphy, Esq., F.G.S. 



[Abstract.] 



The purpose of this paper was to show that " volcanic action is not 

 the cause, but the effect, of secular changes of level ; and secular 

 changes of level are due to the subsidence of the surface of the 

 interior, as the interior contracts in cooling." Change of level is a 

 differential action, and consequently cannot be due to the cooling of 

 a sphere by radiation into space. Volcanic action cannot be due to 

 a spontaneous outburst of the expansive force of the earth's internal 

 heat ; for this could not bui'st through a crust once formed by cool- 

 ing. Changes of level and volcanic action were explained as fol- 

 lows : — The interior of the earth is constantly cooling, and as it 

 cools must contract ; but the cold surface-strata cannot contract 

 with it ; and as their weight keeps them in contact with the core, 

 they are compelled to form ridges like those on the skin of an apple 

 which shrinks in drying. When such a ridge rises into an arch, 

 the hot matter below rises and fills the arch, forming the igneous 

 core of a mountain-chain. Volcanoes are formed when in these fold- 

 ings the surface is broken through, so as to liberate the expansive 

 force of the internal heat. Darwin has shown, in his work on Vol- 

 canic Islands, that volcanoes are formed only in regions which are 



