172 
THE TERTIARY VOLCANOES 
BOOK VIII 
period in the geographical history of the British Islands. But I have no 
evidence that they were restricted to any part of that period. On the 
contrary, there is every reason to consider the uprise of the earliest and 
latest dykes to have been separated by a protracted interval. That they 
do not all belong to one epoch has been already indicated, and may 
now be more specially proved. 
The intersection of one dyke by another furnishes an obvious criterion 
of relative age. Maeculloch drew attention to this test, and stated that it 
had enabled him to make out two distinct sets of dykes in Skye and Bum. 
But he contessed that it failed to afford any information as to the length of 
the interval ot time between them. 1 It is not always so easy as might be 
thought to make sure which of two intersecting dykes is the older. As 
was explained in Chapter vi. (vol. i. p. 81), we have to look for the finer- 
grained marginal strip at the edge of a dyke, which, where traceable across 
another dyke, marks at once their relative age. The cross joints of the two 
dykes also run in different directions. Beference may again be made to the 
illustration given in dig. 253 where three distinct groups of dykes intersect 
each other as they traverse the Lias limestones of Skye. The chilled edges 
and the different arrangement of joints mark these dykes out from each 
other, while the order in which they cross each other furnishes a clue to 
their relative age. If from such sections, repeated in different parts of a 
district, certain persistent petrographical characters can be ascertained to 
distinguish each particular system of dykes, a guide may thereby be 
obtained for the chronological grouping of the intrusions even where 
evidence of actual intersection is not visible. In the case just cited from 
Skye, the later north and south dykes are characterized by their lines of 
vesicular cavities and by the large porphyritic felspars which they contain. 
It is obvious, however, that although sections of this kind suffice to 
prove the dykes to belong to distinct periods of intrusion, no longer 
interval need have elapsed between their successive production than was 
required for the solidification and assumption of a joint-structure by an older 
dyke before a younger broke through it. They may both belong to one 
biief period of volcanic activity. But when we pass to a series of dykes 
traversing a considerable district of country, and find that those which run 
m one direction are invariably cut by those which run in another, the 
inference can hardly be resisted that they do not belong to the same period 
of eruption, but mark successive epochs of volcanic energy. An excellent 
example of this kind of evidence is furnished by Mr. Clough from Eastern 
Aigyleshire. The east and west dykes iii that district are undoubtedly 
older than those which run in a N.N.W. direction (Eig. 25 7). 2 The latter 
are by far the most abundant, and are on the whole much narrower, less 
persistent, and finer in grain. On the opposite coast of the Clyde, a similar 
double set of dykes may be traced through Eenfrewshire, those in an east 
1 Trans. Geol. Soc. iii. p. 75. 
- As already stated, Mr. Clough and also Mr. Gunn are inclined to separate these older east 
and west dykes from the Tertiary series and to regard them as probably of late Paleozoic age. 
