64 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



In the first place, the dykes intersect nearly the whole range of the geological 

 formations of the British Islands. In the Outer Hebrides and north-west Highlands, 

 they rise through the most ancient Archaean gneisses, and through the red (Torridon) 

 sandstones, which may be older than any of the Cambrian rocks of Wales. In the south 

 of Scotland and north of England, they traverse the various subdivisions of the Lower 

 and Upper Silurian system. In the southern Highlands, they pursue their course across 

 the gnarled and twisted schists of the younger crystalline series. In the basins of the 

 Tay, Forth, and Clyde they cross the plains and ridges of the Old Red Sandstone, with 

 its deep pile of intercalated volcanic rocks. In central Scotland, and the northern 

 English counties, they occur abundantly in the Carboniferous system, and have destroyed 

 the seams of coal. In Cumberland and Durham, they traverse the Permian and Trias 

 o-roups. In Yorkshire, and along the west of Scotland, they are found running 

 through Jurassic strata. In Antrim, they intersect the Chalk. Both in the north of 

 Ireland, and all through the chain of the Inner Hebrides, they abound in the great 

 sheets and bosses of Tertiary volcanic rocks. These are the youngest formations 

 through which they rise. But it is deserving of note, that they intersect every great 

 group of these Tertiary volcanic products, so that they include in their number some 

 of the latest known manifestations of eruptive action in the geological history of 

 Britain. 



In the second place, in ranging across groups of rock belonging to such widely 

 diverse periods, the dykes must necessarily often pass abruptly from one kind of material 

 and geological structure to another. But, as a rule, they do so without any sensible 

 deviation from their usual trend, or any alteration of their average width. Here and 

 there, indeed, we may observe a dyke to follow a more wavy or more rapidly sinuous or 

 zig-zag course in one group of rocks than in another. Yet, so far as I have myself 

 been able to observe, such sinuosities may occur in almost any kind of material, 

 and are not satisfactorily explicable by any difference of texture or arrangement 

 in the rocks at the surface. No dyke traverses a greater variety of sedimentary forma- 

 tions than that of Cleveland. In the eastern part of its course, it rises through all the 

 Mesozoic beds up to the Cornbrash. Further west it cuts across each of the different 

 subdivisions of the Carboniferous system ; and, of course, it must traverse all the older 

 formations which underlie these. But the occasional rapid changes noticeable in its 

 width and direction cannot be referred to any corresponding structure in the surrounding 

 rocks. The Cheviot dyke crosses from the Carboniferous area of Northumberland into 

 the Upper Silurian rocks and Lower Old Red Sandstone volcanic tract of the Cheviot Hills. 

 It then strikes across the Upper Old Red Sandstone of Roxburghshire, and still maintain- 

 ing the same persistent trend, sweeps westward into the Lower Silurian rocks of the 

 Southern Uplands. Though liable to occasional deviations, these do not seem to have 

 reference to any visible change of structure in the adjacent formations. Again, some of 

 the great dykes at the head of Clydesdale furnish striking illustrations of entire 

 indifference to the nature of the rock through which they run. Quitting the Lower 



