CHAP. XXXV 
ORIGIN AND HISTORY OF THE DYKES 
177 
the protracted Tertiary volcanic period. One of the most instructive lessons 
in this respect is furnished by the huge eruptive masses of gabbro and grani- 
toid 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 youngest dykes 
generally maintain the usual north-westerly trend across the thickest masses 
of the granophyre. Thus we perceive 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 some fissures were repeatedly reopened and 
admitted a new ascent of molten magma between their walls. 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 Skye and 
the basin of the Clyde, we meet with proofs of a deviation from the normal 
trend. But it is remarkable that dykes which pierce the latest 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 are developed. 
Hopkins, many years ago, discussed from the mathematical side the cause of 
the production of such fissures . 1 He assumed the existence of some eleva- 
tory force acting under considerable areas of the earth’s crust at any assign- 
able depth, either with uniform intensity at every point or with a some- 
what 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.” 2 He showed that such an upheaving force would pro- 
duce in the affected territory a system of parallel longitudinal 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 to increase down- 
wards ; that continued elevation might increase these fissures, but that new 
fissures in the same direction would not arise in the separated blocks which 
would now be more or less independent of each other ; that subsequent sub- 
sidences would give rise to transverse fissures, and by allowing the separated 
blocks to settle down would cause irregularities in the width of the great 
parallel fissures. He considered also the problem presented by those cases 
where the ruptures of the terrestrial crust have been filled with igneous 
1 Cambridge. Phil. Trans, vi. (1835), p. 1. 2 Ibid. p. 10. 
VOL. II -v 
