68 TROCEEBINGS OF THE GEOLOGICAL SOCIETY. 



US in possession of some important information regarding the 

 nature and history of the gneiss *. 



With the possible exception of a strip of ground in the Gairloch 

 district, which inckides graphite-schist, garnetiferous mica-schist, 

 limestone, and a few other remarkable rocks, no portion of the funda- 

 mental gneiss has anywhere yielded a trace of materials that can be 

 supposed to be of sedimentary . origin. Everywhere the rock is 

 thoroughly crystalline, and presents no structure that in any way 

 suggests an alteration of clastic constituents. Here and there it can 

 be traced into bands and bosses which, being either non-foliated 

 or foliated only in a slight degree, present the ordinary characters of 

 true eruptive masses. In Sutherland and Eoss-shire these amor- 

 phous patches occur abundantly. Their external margins are not 

 well defined, and they pass insensibly into the ordinary gneiss, the 

 dark basic massive rocks shading off into the coarse basic gneisses, 

 and the pegmatites of quartz and felspar which traverse them 

 merging into bands of grey quartzose gneiss. 



So far, therefore, as present knowledge goes, the Lewisian gneiss 

 of the North-west Highlands of Scotland was originally a mass of 

 various eruptive rocks. It has subsequently undergone a succes- 

 sion of deformations from enormous stresses within the terrestrial 

 crust, which have been investigated with great care by my colleagues 

 of the Geological Survey. But it presents structures which I 

 venture to think are original, or at least belong to the time of 

 igneous protrusion before the deformations took place. The alter- 

 nation of rocks of different petrographical constitution suggests a 

 succession of extravasations of eruptive materials, though it may be 

 impossible now to determine the order in which these followed each 

 other. In the feebly foliated bands and bosses there is a parallel 

 arrangement of their constituent minerals or of fine and coarse 

 crystalline layers which recalls sometimes very strikingly the flow- 

 structure of rhyolites and other lavas. This resemblance, it will be 

 remembered, was strongly insisted on by Poulett Scrope, who believed 

 that the laminar structure of such rocks as gneiss and mica-schist 

 was best explained by the supposition of the flow of a granitic 

 magma under great pressure within the earth's crust t. Por my 

 own part I must confess that the conviction has grown upon me that 

 these parallel structures do, in some cases, reaUy represent traces of 



* See the Eeport of this Survey work by Messrs. Peach, Home, Gunn, Clough, 

 Cadell, and Hinxman, Quart. Journ. Geol. Soc. vol. xliv. (1888) pp. 378-441. 

 t ' VolcanoeB, 'pp. 140, 283, 299. 



