Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/220

 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. Carruthers 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 Forbes agreed that the crumpling of the strata was not due to the intrusion of any eruptive rock. He completely disagreed with Prof. Ramsay and the author as to the origin of granite, and maintained that, in the sedimentary rocks traversed by the granite, 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 approaching gradually to some portion of the surface.

Prof. Ramsay, 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 with 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 burst through a crust once formed by cooling. Changes of level and volcanic action were explained as follows : — 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 foldings the surface is broken through, so as to liberate the expansive force of the internal heat. Darwin has shown, in his work on Volcanic Islands, that volcanoes are formed only in regions which are