CHAP. XXXV 
METAMORPHISM BY DYKES 
165 
These, however, are the extremes of contact -metamorphism by the 
Tertiary basic dykes. A geologist visiting the Liassic shores of Strath in 
Skye will not fail to be surprised at the very slight degree of alteration in 
circumstances where he would have expected to find it strongly pronounced. 
The dark shales, though ribbed across with dykes, are sometimes hardly 
even haidened, and at the most are only indurated from an inch or two to 
about two feet. These baked bands project above the rest of the more 
easily denuded shales, and so adhere to the dykes as almost to seem part of 
them. Again the limestones, where traversed by dykes some distance 
apart, are not rendered in any appreciable degree more crystalline even 
up to the very margin of the intrusive rock. Where the igneous material 
has been thrust between the strata in sills, it has produced far more general 
and seiious metamorphism than when it occurs in the form of single 
dykes. The famous rock of Portrush, already referred to as having 
been once gravely cited as an example of fossiliferous basalt, is a 
good illustration of the way in which Lias shale is porcellanized when 
the intruded igneous material has been thrust between the planes of 
bedding. 
In the West of Scotland, where dykes are so abundantly developed, con- 
siderable differences can be observed between the amount of metamorphism 
superinduced by adjacent dykes which may be of the same thickness, and 
cut through the same kind of strata. Such variations have not probably 
arisen from differences in the temperature of the original molten rock. 
Perhaps they are rather to be assigned to the length of time occupied by 
the ascent of the lava in the fissure. If, for instance, the fissure opened to 
the surface and discharged lava there, the rocks of its walls would be exposed 
to a continuous stream of molten rock as long as the outflow lasted. They 
would thus have their temperature more highly raised, and maintained 
at such an elevation for a longer time than where the magma, at once 
arrested within the fissure, immediately proceeded to cool and consolidate 
there. It would be an interesting and important conclusion if we could, 
irom the nature or amount of their contact-metamorphism, distinguish those 
dykes which for some time served as channels for the discharge of lava 
above ground. 
Some dykes which have caught up fragments of older rocks in their 
ascent have exercised a considerable solvent action on these inclusions. 
Examples of this feature have already been cited from Skye, where they 
have been studied by Mr. Harker (pp. 129, 163). 
In connection with the metamorphism superinduced by dykes, reference 
laay again be made to the alteration which they themselves undergo where 
icy have invaded a carbonaceous shale or coal. The igneous rock, as we 
ave seeu > i° ses its dark colour and obviously crystalline structure, and be- 
comes a pale yellow or white, dull, earthy substance, or “ white trap.” The 
^ lem ical changes involved in this alteration have been described by Sir J. 
owthian Bell. 1 Dr. Steelier has also discussed the alterations traceable by 
1 Proc. Roy. Soc. xxiii. (1875), p. 543. 
