﻿hi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



rocks, it is obvious that it must necessarily have been formed by the 

 detritus of an original imstratified surface-rock, which had been ex- 

 posed to the wearing effects of atmospheric action and of waves acting 

 upon a shore. The detritus, in order to form stratification, must 

 have been spread gradually over the solid bottom of a deep sea. 

 The generally prevalent theory assumes that the altering heat was 

 communicated through the rock on which the sediment rested, that 

 is, the sea-bottom. Now unless we assume that the sea was in a 

 state of ebullition, the bottom rock must have been cold, by being in 

 immediate contact with cold water ; for so soon as the sea- water came 

 in contact with a heated mass, immediately there would be produced 

 two continuous currents, of ascending warm and descending cold 

 water, until the whole sea was raised to a high temperature. By 

 the pressure of the superincumbent water, the boiling-point in the 

 lower depths would be greatly raised, and the constant production 

 and upward direction of high-pressure steam would cause such a 

 turmoil in the sea that no tranquil deposition of sediment to form 

 stratification could possibly take place. If the source of heat were 

 local, the colder water from the adjoining parts of the sea woidd rush to 

 the heated parts. If the bottom rock did not become heated until after 

 the sediment had accumulated to a great thickness, it is obvious, 

 from the very slow conducting power of rocks, that the lower parts, if 

 brought by the heat into such a softened state as to acquire a crystal- 

 line structure, must be very different in nature from the upper parts 

 of the deposit, — that, in short, there would be a gradual change in 

 the texture of the rock upwards — a difference which has nowhere 

 been observed. A similar difficulty attends the hypothesis in other 

 cases besides that of gneiss, when metamorphism is considered to 

 have been produced by sedimentary deposits having been acted upon 

 by a highly heated hypogene rock at the bottom of a sea. 



As gneiss has never been seen to contain an undoubted fragment 

 of a pre-existent rock, nor any trace of an organism, its being held 

 to be an altered sedimentary deposit woidd seem to rest, first, on its 

 schistose and bedded structure, and, secondly, upon the extreme im- 

 probability of an eruptive rock having spread over vast regions with 

 that structure. As regards schistose and bedded structure, that is 

 of itself no conclusive proof. Gneiss is essentially composed of the 

 same materials as eruptive granite, and there are numberless in- 

 stances, on a great scale, of a gradual passage from coarse-grained 

 granite into schistose gneiss. Some of these I will quote. 



Dr. MacCulloch, in his ' Description of the Western Islands,' when 

 treating of gneiss, which prevails in the Northern Hebrides, observes* 

 that there are two principal varieties, the one of a granitic, the 

 other of a schistose structure ; that the gneiss of these islands (Flan- 

 nan Isles) is in general composed of quartz, felspar, and mica ; that it 

 presents as usual many varieties, and among the rest, one which can- 

 not be distinguished from common granite ; that this consists of an 

 equal mixture of flesh-coloured felspar, quartz, and but little mica, 

 forming beds among the rest of the rock ; that there is not a trace 

 * Vol. i. pp. 202 & 226. 



