So far as I am aware, no undoubted vents of the age of the Lower Old Red Sandstone have been detected among the high grounds of the Highlands on the one hand, or among the Silurian uplands on the other, although a fringe of the lavas may be traced here and there along the base of the hills.[325] In some cases, doubtless, the position of the valleys may have been determined by lines of fault that might well serve as lines of relief along which volcanic vents would be opened. But in many instances it can be proved that, though the vents have risen in valleys and low grounds, they have not selected lines of fault visible at the surface, even when these existed in their neighbourhood. Any fissures up which the volcanic ejections made their way must have lain at great depths beneath the formations that now form the surface rocks.

[325] Certain remarkable necks of breccia have been detected by Mr. J. R. Dakyns rising through the schists at the upper end of Loch Lomond; but there is not sufficient evidence to connect them with the volcanic series of the Lower Old Red Sandstone. Some of the younger granite bosses are not improbably to be referred to this volcanic series. The latest granites of the eastern Grampians, as already stated, have lately been found by Mr. Barrow cutting the band of probably Lower Silurian strata along the southern border of the Highlands. Those of Galloway are younger than the Upper Silurian formations, which they invade, and older than the conglomerates of the Upper Old Red Sandstone, which contain pebbles of them. These eruptive bosses will be further discussed in the sequel.

ii. CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES

A general summary of the petrographical characters of the igneous rocks of the Lower Old Red Sandstone may here find a place. Further details will be given in the account of "Lake Caledonia," which is the typical area for them; but, on the whole, the prevailing types in one region are found to be repeated in the others.

1. Bedded Lavas.—Beginning with the lavas which were poured out at the surface, we have to notice a considerable range of chemical composition among them, although, as a rule, they are characterized by general similarity of external appearance. At the one end, come diabases and other ancient forms of basalt or dolerite, wherein the silica percentage is below or little above 50. By far the largest proportion of the lavas, however, are porphyrites or altered andesites, having about 60 per cent of silica. With these are associated lavas containing more or less unstriped felspar and a somewhat higher proportion of silica, which may be grouped as trachytes, though no very sharp line can be drawn between them and the andesites. In the Pentland Hills, and some other areas, orthophyres flowed out alternately with the more basic lavas, and were associated with felsitic tuffs and breccias.

It is noteworthy that the lava-sheets of the Lower Old Red Sandstone, if we consider the character of the prevalent type, hold an intermediate grade between the average chemical composition of those of Silurian and of those of later Carboniferous time. On the one hand, they rarely assume the character of thoroughly acid rocks, like the nodular rhyolites of the Bala and Upper Silurian series;[326] on the other hand, they seldom include such basic lavas as the basalts, so common among the puy-eruptions of the Carboniferous system, and never, so far as I know, contain varieties comparable to the "ultra-basic" compounds which I shall have occasion to allude to as characteristic of a particular volcanic zone in that system.

[326] The only examples known to me are those of Benaun More and other hills in County Kerry.

(a) The Diabase-lavas are typically developed in the chain of the Pentland Hills, where they form long bands intercalated between felsitic tuffs—a remarkable association, to which I shall make more detailed reference in a later chapter. They range in texture from a compact dark greenish base to a dull earthy amygdaloid. One of their most remarkable varieties is a fine-grained green porphyry, with large flat tabular crystals of plagioclase arranged parallel to the direction of flow (Carnethy Hill). Most of them, however, are more or less amygdaloidal, and some of them (Warklaw Hill) strongly so. The following analyses, made in the laboratory of the Royal School of Mines under the direction of Prof. E. Frankland, show the chemical composition of some of the diabases of the Pentland Hills:[327]—

[327] For analyses of some Shetland diabases of Old Red Sandstone age, see Mr. R. R. Tatlock, Trans. Roy. Soc. Edin. vol. xxxii. (1887), p. 387.

SiO2Al2O3Fe2O3FeOCaOMgOK2ONa2OH2OP2O5CO2
Carnethy Hill[328]51·1622·272·944·025·613·462·422·583·420·481·28
Buiselaw.
Sp. grav.
2·80.		Soluble
in HCl		...1·301·531·142·430·98.........0·32...
Insoluble
in HCl		52·0017·467·85...6·801·061·664·172·68... ...
Warklaw
Hill.
Sp. grav.
2·77.		Soluble
in HCl		...5·237·32...7·883·65.........0·125·01
Insoluble
in HCl		47·7713·080·84...4·070·301·172·302·48......