1. Basalt, Dolerite and Andesite Dykes.--To the field-geologist, who regards merely their external features, the Tertiary 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, amygdales, glassy portions and other points of structure. But there is seldom any difficulty in perceiving that they generally belong 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. There appears to be reason to believe that, when the petrography of the dykes is more minutely studied, marked differences of material will be found to denote distinct periods of eruption. Already Mr. A. Harker of the Geological Survey, who is engaged in mapping the interesting and complicated district of Strath in Skye, has observed that the dykes which are older than the great granophyre bosses of that tract may be distinguished from those which are later than these protrusions. The older basic dykes are not conspicuously porphyritic, are frequently marked by a close-grained margin or even with a veneer of basalt-glass, sometimes have an inclination of as much as 45°, are occasionally discontinuous, and not infrequently branch or send out veins. The younger dykes, on the other hand, as will be more particularly noticed in the following chapter, are distinguished by the frequent and remarkable character of their porphyritic inclusions, by the presence of foreign fragments in them, by the greater perfection of their jointing, and by their seldom departing much from the vertical.[163] They are likewise often markedly acid in composition, including such rocks as granophyre, felsite and pitchstone. [Footnote 163: In the Blath Bheinn group of gabbro-hills, however, it is the youngest dykes which have been found by Mr. Harker to possess the lowest hade.]