DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 65 



Silurian uplands, they keep their line across Upper Silurian, 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 Red Sand- 

 stone against the Lower 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 boundary-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. 



No one can contemplate these repeated examples of an entire waDt of connection 

 between the dykes and the nature and arrangement of the rocks which they traverse 

 without being convinced that the lines of vent 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 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, I can almost reckon 

 on my fingers the number of instances where dykes have availed themselves of lines of 

 fault. Some of these will be immediately cited. To whatever cause we may ascribe the 

 rupture of the solid crust of the earth, which allowed of 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. 7 and 8). Besides the localities already cited, 

 mining operations both for coal and for the Liassic ironstone have proved over a wide 

 area that the dyke has not risen along a line of fault. Again, in Skye, Eaasay, Eigg, and 

 other parts of the west coast, where Jurassic strata and the horizontal basalts of the 

 plateaux are plentifully cut through by dykes, the same beds may be seen on the same 

 level on either side of them. 



In the fifth place, while complete indifference to geological structure is the general 



