﻿ANNIVERSARY ADDRESS OF THE PRESIDENT. lvil 



of laminar tendency in their structure, and therefore, although they 

 may be considered as forming portions of the gneiss beds, they pre- 

 sent, in strictness, an example of bedded granite. He then goes 

 on to say that the limits which separate gneiss from granite are 

 evanescent, and that of this perpetual gradation there is scarcely a 

 mile of the survey of those islands which does not offer an example ; 

 that the granitic subdivision prevails, and is characterized not only 

 by a large grain and imperfectly foliated structure, but by frequent 

 transitions into granite, from which, when in detached specimens, it 

 cannot be distinguished. 



As instances out of our own country of this intimate relation between 

 varieties of gneiss and granite, I may give the following : — Carl von 

 Eoemer, as quoted by Naumann*, while describing the northern side 

 of the central granite of Siberia, mentions an extensive tract of granit- 

 gneiss in which a laminar and bedded rock repeatedly alternates with 

 one of granular and massive structure without lamination ; and in like 

 manner, in Podolia, granite and gneiss form a massive, intermixed, 

 and compact whole, demonstrating a contemporaneity and similarity of 

 origin of both. Beudant states that in Hungary gneiss and granite 

 appear always together, and that they occur not only in alternating 

 beds, but as one and the same mass. Elie de Beaumont t states that a 

 passage between granite and gneiss, gneiss and mica-schist, and even 

 between granite and mica-schist, is so often observed as to prove, in 

 such cases, their common origin ; that there are instances in which 

 it is evident that gneiss must have been an eruptive rock, which, 

 after its eruption, had been drawn out so as to assume a schistose or, 

 rather, a fibrous structure, so that it is often difficult to distinguish 

 between the two kinds, — that is, between eruptive and metamor- 

 phic gneiss. M. Delesse says, that gneiss forms a transition be- 

 tween stratified and eruptive rocks ; that its mineralogical composi- 

 tion, as well as its mode of occurrence, unite it in a manner the most 

 intimate with granite, and that its origin is evidently the same+. 

 The section of granite and gneiss at Jaegersborg, in Norway, given 

 by Mr. David Forbes §, proves indisputably, I think, that there the 

 two rocks must have had a common origin, and that it is not con- 

 ceivable that this gneiss can be an altered sedimentary rock. 



When stratified gneiss contains, as it often does, a variety of ac- 

 cessory simple minerals, and metamorphism by contact likewise pro- 

 duces them in the traversed rock, we have a complication of chemi- 

 cal action for a right understanding of which we must look to syn- 

 thetic experiment. Thus we know [| that, in limestones especially, 

 a great variety of minerals are often produced by the contact of 

 granite, among which garnet, idocrase, hornblende, wollastonite, 

 epidote, talc, chlorite, and zeolites are the most common. Now to 

 produce these eight minerals, there must be derived from the granite 



* Lehrbuch der Geognosie, 1st eel. vol. ii. p. 84. t Loc. cit. 



\ Etudes, &c, 4to, p. 203. 



§ Quart. Journ. Geol. Soc, vol. xv. p. 173. 



|| Daubree, Etudes et Experiences synthetiques, p. 57. 



