DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 71 



of lines, and are distinct from any other architectonic feature in the geology of the 

 country. They did not arise therefore by a mere renewal of some previous order of 

 disturbances, but were brought about by a new set of movements to which it is 

 difficult to find any parallel in the earlier records of the region.* 



We have further to remember that the fissures were not produced merely by one great 

 disturbance. The evidence of the dykes proves beyond question that some of them are 

 earlier than others, and hence that the cause to which the fissures owed their origin came 

 into operation more than once during the protracted Tertiary volcanic period. One of 

 the most instructive lessons in this respect is furnished by the huge eruptive masses of 

 gabbro and granitoid rocks in Skye. These materials have been erupted through the 

 plateau-basalts. The granitoid bosses are the younger protrusions, for they send veins 

 into the gabbros ; but their appearance was later than that of some of the dykes and 

 older than that of others. Nevertheless, the latest dykes maintain the usual north-westerly 

 trend across the thickest masses both of the gabbro and the granophyre. Thus we learn 

 that, even after the extrusion of thousands of feet of such solid crystalline igneous rocks, 

 covering areas of many square miles, the Assuring of the ground was renewed, and rents 

 were opened through these new piles of material. From the evidence of the dykes also, 

 we learn that the general direction of the fissures remained from first to last tolerably 

 uniform. Here and there indeed, where one set of dykes traverses another, as in the 

 basin of the Clyde, we meet with proofs of a deviation from the normal trend. But it is 

 remarkable that the very youngest dykes which pierce the eruptive bosses of the Inner 

 Hebrides rose in fissures that were opened in the normal north-westerly line through 

 these great protrusions of basic and acid rock. 



Such a gigantic system of parallel fissures points to great horizontal tension of the 

 terrestrial crust over the area in which they were developed. Hopkins, many years ago, 

 discussed from the mathematical side the cause of the production of such fissures.t He 

 assumed the existence of some elevatory force acting under considerable areas of the 

 earth's crust at any assignable depth, either with uniform intensity at every point, or with 

 a somewhat greater intensity at particular points. He did not assign to this force any 

 definite origin, but supposed it "to act upon the lower surface of the uplifted mass 

 through the medium of some fluid, which may be conceived to be an elastic vapour, or, 

 in other cases, a mass of matter in a state of fusion from heat." J He showed that such 

 an upheaving force would produce in the affected territory a system of parallel longi- 

 tudinal fissures, which, when not far distant from each other, could only have been 

 formed simultaneously, and not successively ; that each fissure would begin not at the 

 surface, but at some depth below it, and would be propagated with great velocity ; that 

 there would be more fissures at greater than at lesser depths, many of them never reaching 

 the surface ; that they would be of approximately uniform width, the mean width tending 



* The only other known example of such a dyke-structure is that of the Pre-Cambrian series of dykes in the 

 Archaean gneiss of Sutherland. 



t Cambridge Phil. Trans., vi. (1835) p. 1. % Op. cit, p. 10. 



