DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 35 



§ 3. Nature of Component Eooks. 



The Tertiary dykes of Britain include representatives of two very distinct groups of 

 igneous rocks. The vast majority of them are basic compounds, belonging to the family 

 of the pyroxenic lavas, which, where the percentage of silica is relatively high, are known 

 as andesites, and where it is relatively low, have been variously styled basalts, dolerites, 

 melaphyres, or diabases. It is to these basic dykes that the general descriptions of the 

 present section exclusively refer. The second class is composed of an acid rock, either 

 more or less crystalline, such as felsite, quartz-porphyry, rhyolite or trachyte ; or vitreous, 

 in the form of pitchstone. These acid dykes or veins, though extremely abundant at a 

 few localities, are on the whole rare. They will be described by themselves in subsequent 

 pages (p. 175). 



To the field geologist, who has merely their external features to guide him, the 

 ordinary Tertiary basic dykes present a striking uniformity in general petrographical 

 character. They vary indeed in fineness or coarseness of texture, in the presence or 

 absence of porphyritic crystals, amygdules, glassy portions, and other points of structure. 

 But there is seldom any difficulty in perceiving that they are basic rocks belonging to 

 one or other of the types of the basalts, dolerites, diabases, or andesites. This sameness 

 of composition, traceable from Yorkshire to Skye and from Donegal to Perthshire, is 

 one of the strongest arguments for referring this system of dykes to one geological period. 

 At the same time, there are enough of minor variations and local peculiarities to afford 

 abundant exercise for the observing faculties alike in the field and in the study, and to 

 offer materials for arriving at some positive conclusions regarding the geological processes 

 involved in the uprise of the dykes. 



1. External Characters. — As regards the grain of the rock, every gradation may be 

 found, from a coarsely crystalline mass, in which the component minerals are distinctly 

 traceable with the naked eye, to a black lustrous basalt-glass. Each dyke generally 

 preserves the same character throughout its extent. As a rule, broad and long dykes are 

 coarser in grain than narrow and short ones. For the most part, there runs alongside 

 each side of a dyke a selvage of finer grain than the rest of the mass. This marginal strip 

 varies in breadth from an inch or less up to a foot or more, and obviously owes its origin 

 to the more rapid chilling of the molten rock along the walls of the fissure. It usually 

 shades away imperceptibly into the larger-grained inner portion. Even with the naked 

 eye, its component materials can be seen to be more finely crystalline than the rest of the 

 dyke, though where dispersed porphyritic felspars occur they are usually as large in the 

 marginal strip as in any other part of a dyke. 



This finer-grained external band, so distinctive of an eruptive and injected rock, is of 

 great service in enabling us to trace dykes when they traverse other dykes or masses of 

 igneous rock of similar characters to their own. When one dyke crosses another, that 

 which has its marginal band of finer grain unbroken must obviously be the younger of 

 the two. 



