142 
THE TERTIARY VOLCANOES 
BOOK VIII 
6. INTERRUPTIONS OF LATERAL CONTINUITY 
In tracing the great solitary 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 disappearance is not always explicable by the 
depth of the cover of superficial accumulations ; for it may be observed 
over ground where the naked rocks come almost everywhere to the surface, 
and where, therefore, if the conspicuous material of the dykes existed, it 
could not fail to be found. No dyke supplies better illustrations of this 
discontinuity than that of Cleveland. Traced north-westward across the 
Carboniferous tracts that lie between the mouth of the Tees and the A r ale 
of the Eden, this dyke disappears sometimes for a distance of six or eight 
miles. In the mining ground round the head of the South Tyne the rocks 
are bare, so that the absence of the dyke among them can only be accounted 
for by its not reaching the surface. Yet there can be no doubt that the 
various separated exposures, which have the same distinctive lithological 
characters and occur on the same persistent line, are all portions of one dyke 
which is continuous at some depth below ground. We have thus an indi- 
cation of the exceedingly irregular upward limit of the dykes, as will be 
more particularly discussed further on. 
But there are also instances where the continuity is interrupted and 
then resumed on a different line. One of the best illustrations of this 
character is supplied by the large dyke which rises through the hills about 
a mile south of Linlithgow and runs westward across the coal-field. At 
Blackbraes it ends off in a point, and is not found again to the westward in 
any of the coal-workings. But little more than a quarter of a mile to the 
south a precisely similar dyke begins, and strikes westward parallel to the 
line of the first one. The two separated strips of igneous rock overlap 
each other for about three-quarters of a mile. But that they are merely 
interrupted portions of what is really a single dyke can hardly be ques- 
tioned. A second example is furnished by another of the great dykes of 
the same district, which after running for about twelve miles in a nearly 
east and west direction suddenly stops at Chryston, and begins again in the 
same direction, but on a line about a third of a mile further north. Such 
examples serve to mark out irregularities in the great fissures up which the 
materials of the dykes rose. 
7 . LENGTH 
In those districts where the small and crowded dykes of the gregarious 
type are developed, one cannot usually trace them for more than a short 
distance. The longest examples known to me are those which have been 
mapped with much patience and skill by Mr. Clough in Eastern Argyle- 
shire. Some of them he has been able to track over hill and valley for 
four or five miles, though the great majority are much shorter. In Arran 
and in the Inner Hebrides, it is seldom possible to follow what we can be 
sure is the same dyke for more than a few hundred yards. This difficulty 
