CHAP. XXXV 
DYKES AND GEOLOGICAL STRUCTURE 
1 6 7 
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 (Lewisian) 
gneisses, through the red pre-Cambrian (Torridon) sandstones, and through 
the oldest members of the Cambrian system. In the southern Highlands, 
they pursue their course across the gnarled and twisted schists of the 
younger crystalline (Dalradian) series. In the South of Scotland and North 
of England, they traverse the various subdivisions of the Lower and Upper' 
Silurian rocks. In the basins of the Tay, Forth, and Clyde they cross the 
plains and ridges of the Old Bed Sandstone, with its deep pile of intercalated 
volcanic material. 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 groups. 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 the latest 
known manifestations of eruptive action in the geological history of 
Britain . 1 
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 sedi- 
mentary formations than that of Cleveland. In the eastern part of its 
course, it rises through all the Mesozoic groups 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 forma- 
tions which underlie these. But the occasional rapid changes noticeable in 
its width and direction do not seem to be referable 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 Bed Sandstone volcanic tract of the Cheviot Hills. It then 
strikes across the Upper Old Bed Sandstone of Boxburghshire, and still 
maintaining the same persistent trend, sweeps westward into the intensely 
plicated Silurian rocks of the Southern Uplands. Its occasional deviations 
lave no obvious reference to any visible change of structure in the adjacent 
1 They have not been found cutting the pitchstone-lava of the Scuir of Eigg. 
