THE TERTIARY VOLCANOES 
BOOK VIII 
1 68 
formations. Again, some of the great dykes at the head of Clydesdale 
furnish striking illustrations of entire indifference to the nature of the ^ock 
through which they run. Quitting the Silurian uplands, they keep their line 
across Old Eed Sandstone and Carboniferous rocks, and through large masses 
of eruptive material. 
In the third place, not only are the dykes not deflected by great 
diversities in the lithological character of the rocks which they traverse, 
’they even cross without deviation some of the most important geological 
features in the general framework of the country. Some of the Scottish 
examples are singularly impressive in this respect. Those which strike 
north-westward from the uplands of Clydesdale cross without deflection the 
great boundary-fault which, by a throw of several thousand feet, brings the 
Lower Old Eed Sandstone against Silurian rocks. They traverse some large 
faults in the valley of the Douglas coal-field, pass completely across the axis 
of the Haughshaw Hills, where the Upper Silurian rocks are once more 
brought up to the surface, and also the long felsite ridge of Priesthill. The 
dykes in the centre of the kingdom maintain their line across some of the 
large masses of igneous rock that protrude through the Carboniferous 
system. Further north, the dykes of Perthshire cut across the great sheets 
of volcanic material that form the Ochil Hills, as well as through the piles 
of sandstone and conglomerate of the Lower Old Eed Sandstone, and then go 
right across the bounclary-fault of the Highlands, to pursue their way in the 
same independent manner through grit, quartzite, or mica-schist, and across 
glen and lake, moor and mountain. 
Ho one can contemplate these repeated examples of an entire want of 
connection between the dykes and the nature and arrangement of the rocks 
which they traverse without being convinced that the lines of rent up which 
the material of the dykes rose were not, as a rule, old fractures in the 
earth’s crust, but were fresh fissures, opened across the course of the older 
dislocations and strike of the country by the same series of subterranean 
operations to which the uprise of the molten material of the dykes was also 
due. 
In the fourth place, the dykes for the most part are not coincident with 
visible lines of fault. After the examination of hundreds of dykes in all parts 
of the country, and with all the help which bare hill-sides and well-exposed 
coast-sections can afford, the number of instances which have been met with 
where dykes have availed themselves of lines of fault is surprisingly small. 
Some of these cases will be immediately cited. To whatever cause we may 
ascribe the rupture of the solid crust of the earth, which admitted the rise 
of molten rock to form the dykes, there can be no doubt that it was not 
generally attended with that displacement of level on one or both sides of 
the dislocation, which we associate with the idea of a fault. Nowhere can 
this important part of dyke-structure be more clearly illustrated than along 
the Cleveland dyke, where the igneous rock rises through almost horizontal 
Jurassic strata and gently inclined Coal-measures (Figs. 241, 242, 243, 244). 
Besides the localities already cited, mining operations both for coal and for 
