CHAP. XXXIV 
BREADTH OF THE DYKES 
141 
are available of obtaining information on the subject — ( a ) from mining 
operations, and (?>) from observations at precipices and between hill-crests 
and valley-bottoms. 
(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. 
(h) 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 many 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. 1 Now and then one of them 
may be observed to die out, but in such cases (which are far from common) 
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 super- 
ficial 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. 
1 This point did not escape tlie attention of that excellent observer, Berger, in his examina- 
1011 °f the dykes in tlie 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 
ong the coast in clifis 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. 
in* p. 227). 
