DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 47 



(a) In the central Scottish coal-field and in that of Ayrshire some large dykes have 

 been cut through at depths of two or three hundred feet beneath the surface. But 

 there does not appear to be any well-ascertained variation between their width so far 

 below ground and at the surface. In not a few cases, indeed, dykes are met with in 

 the lower workings of the coal-pits which do not reach the surface or even the workings 

 in the higher coals. Such upward terminations of dykes will be afterwards considered, 

 and it will be shown that towards its upper limit a dyke may rapidly diminish in width. 



(6) More definite information, and often from a wider vertical range, is to be gathered 

 on coast-cliffs and in hilly districts, where the same dyke can be followed through a 

 vertical range of several hundred feet. But so far as my own observations go, no 

 general rule can be established that dykes sensibly vary in width as they are traced 

 upward. Every one who has visited the basalt precipices of Antrim or the Inner 

 Hebrides, where dykes are so numerous, will remember how uniform is their breadth as 

 they run like ribbons up the faces of the escarpments.* Now and then one of them may 

 be observed to die out, but in such cases (which are far from common among true dykes) 

 the normal width is usually maintained up to within a few feet of the termination. 



All over the southern half of Scotland, where the dykes run along the crests of the 

 hills and also cross the valleys, a difference of level amounting to several hundred feet may 

 often be obtained between adjacent parts of the same dyke. But the breadth of igneous 

 rock is not perceptibly greater in the valleys than on the ridges. The depth of boulder 

 clay and other superficial deposits on the valley bottoms, however, too frequently conceals 

 the dykes at their lowest levels. Perhaps the best sections in the country for the study 

 of this interesting part of dyke-structure are to be found among the higher hills of the 

 Inner Hebrides, such as the quartzites of Jura and the granophyres and gabbros of Skye. 

 On these bare rocky declivities, numerous dykes may be followed from almost the sea- 

 level up to the rugged and splintered crests, a vertical distance of between 2000 and 

 3000 feet. The dykes are certainly not as a rule sensibly less in width on the hill tops 

 than in the glens. So far, therefore, as I have been able to gather the evidence, there 

 does not appear to me to be, as a general rule, any appreciable variation in the width of 

 dykes for at least 2000 or 3000 feet of their descent. The fissures which they filled must 

 obviously have had nearly parallel walls for a long way down. 



§ 6. Interruptions of Lateral Continuity. 



In tracing the great single dykes across the country, the geologist is often surprised 

 to meet with gaps, varying in extent from a few hundred feet to several miles, in which 

 no trace whatever of the igneous rock can be detected at the surface. This dis- 

 appearance is not always explicable by the depth of the cover of superficial accumulations; 



* This point did not escape the attention of that excellent observer, Berger, in his examination of the dykes in 

 the north of Ireland. We find him expressing himself thus : — "The depth to which the dykes descend is unknown; 

 and after having observed the sections of a great many along the coast in cliffs from 50 to 400 feet in height, I have not 

 been able to ascertain (except in one or two cases) that their sides converge or have a wedgeform tendency" (Trans. 

 Geol. Soc, iii. p. 227). 



