﻿242 Geological Society : — 



earth-movements, and the absence of authentic specimens of the 

 gneiss in the Cretaceous, Eocene, and Miocene conglomerates, renders 

 their late Tertiary age highly probable. 



The nature of the contact-metamorphism and the origin of the 

 gneissic structure are discussed, and a classification offered of the 

 earth-movements in the Cottian Alps. 



February 21st. — Dr. Henry Woodward, F.R.S., President, 

 in the Chair. 

 The following communications were read : — 

 1. ' On the Relations of the Basic and Acid Rocks of the Tertiary 

 Volcanic Series of the Inner Hebrides.' By Sir Archibald Geikie, 

 D.Sc, LL.D., F.R.S., P.G.S. 



After an introductory sketch of his connexion with the inves- 

 tigation of the Tertiary volcanic rocks of Britain, the Author 

 proceeds to describe the structure of the ground at the head of Glen 

 Sligachan, Skye, which has recently been cited by Prof. Judd as 

 affording inclusions of Tertiary granite in the gabbro, and as thus 

 demonstrating that the latter is the younger rock. He first shows 

 that the gabbro, instead of being one eruptive mass, consists of 

 numerous thin beds and sills of different varieties of gabbro, some 

 of which were injected into the others. These various sheets, 

 often admirably banded, can be seen to be truncated by the line of 

 junction with the great granophyre-tract of Glen Sligachan. A large 

 mass of coarse agglomerate is likewise cut off along the same line. 

 These structures are entirely opposed to the idea of the gabbro being 

 an eruptive mass which has broken through the granophyre. They 

 can only be accounted for, either by a fault which has brought the 

 two rocks together, or by the acid rock having disrupted the basic. 

 But there is ample evidence that no fault occurs at the boundary- 

 line. 



The granophyre becomes fine-grained, felsitic, and spherulitic 

 along its margin, where it abuts against the complex mass of 

 basic rocks. These structures continue altogether independent of 

 the varying distribution of the gabbros, and are seen even where 

 the granophyre runs along the side of the agglomerate. Similar 

 structures are of common occurrence along the margins of the 

 granophyre-bosses and sills of the Tuner Hebrides, being found 

 not only next the gabbro, but next the Jurassic sandstones and 

 shales. They are familiar phenomena of contact in all parts of the 

 world, and are sufficient of themselves to show that the granophyre 

 of Skye must be later than the gabbro. 



The Author then describes three conspicuous dykes, from 8 to 

 10 feet broad, which can be seen proceeding from the main body of 

 granophyre and cutting across the banded gabbros. One of these is 

 traceable for more than 800 feet in a nearly straight line. The 

 material composing these dykes is identical with that constituting 

 the marginal portion of the granophvre-mass. It presents the most 

 exquisite flow-structure, with abundant rows of spherulites. The 



