394 



SIR A. GEIKIE ON THE TERTIARY 



[May 1896, 



dykes. On either side of this acid centre a narrow basalt-dyke 

 intervenes as a wall, next to the Torridon Sandstone which here 

 forms the country-rock. 



In this instance, and generally throughout the district, there is 

 nothing to indicate that the different bands of the dyke have any 

 relation to each other 



FU 



29. — Compound dyke. Market 

 Stance, Broadford, Skye. 



Granophyre 



Basalt 



don sandstone. 



as connected uprises of 

 material from the same 

 original magma which was 

 undergoing a process of 

 differentiation beneath the 

 terrestrial crust. On the 

 contrary, the several parts 

 of each dyke are as distinctly 

 marked off from each other 

 as they could have been 

 had they been injected at 

 widely separated intervals 

 of volcanic activity. 



The same indication of 

 an independent origin is 



displayed by the rocks when they form compound sills, with a 

 thick central sheet of acid material overlain and underlain by some 

 more basic rock. I have shown that the posteriority of the acid 

 sill may sometimes be demonstrated by its sending out veins into 

 the darker sill above or below it. 1 But a more striking proof of 

 the independence of the two kinds of rock may be seen at Suishnish 

 Point, in the Isle of Raasay (fig. 30). Here the Pabba Shales of 

 the Lower Lias (a a) are surmounted by a sheet of granophyre (0), of 

 which the top has been removed by denudation. This rock occupies 

 about 5 square miles in the southern half of the island, where it has 

 recently been mapped by Mr. H. B. Woodward for the Geological 

 Survey. It has been intruded across the Jurassic Series, a large 

 part of its mass coming in irregularly about the top of the thick 

 white sandstones of the Inferior Oolite. But it descends beneath 

 the Secondary rocks altogether, and in some places intervenes 

 between the base of the Infra-Liassic conglomerates and the Torridon 

 Sandstone. 



The central portions of this Eaasay granophyre possess the 

 ordinary structures of the corresponding rocks in Skye. The)?' show 

 a fine crystalline-granular micropegmatitic base, through which 

 large felspars and quartzes are dispersed. But at the upper and 

 under junctions with the sedimentary rocks beautiful spheruliric 

 structures are developed. This is w^ell seen on the shore near 

 the Point of Suishnish, where, below the Liassic limestones, the top 

 of the granophyre appears, and where its bottom is seen to lie on 

 the Torridon Sandstone. 



Where the eruptive rock rests on the Pabba Shales, a basalt- 

 dyke which rises through these strata turns abruptly at the base 

 of the acid rock and then pursues its course to one side as a sill (c) 

 1 Trans. Eoy. Soc. Edinb. vol. xxxv. (1888) p. 174. 



