DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 63 



porcelain-like material. Limestone is changed for a few inches into marble. Sandstones 

 are indurated into a kind of quartzite, sometimes assume a columnar structure (the 

 columns being directed away from the dyke-walls), and for several feet or yards have their 

 yellow or red colours bleached out of them. The granite of Ben Cruachan where quarried 

 on Loch Awe, as I am informed by Mr J. S. Grant Wilson of the Geological Survey, is 

 traversed by a basic dyke, and for a distance of about 20 feet is rendered darker in 

 colour, becomes granular, and cannot be polished and made saleable. 



These, however, are the extremes of contact-metamorphism by the Tertiary basic dykes. 

 Let any geologist visit the Lias district of Skye, and he will not fail to be surprised at the 

 almost entire absence of alteration in circumstances where he would have expected to find 

 it. The dark shales, though ribbed across with hundreds of dykes, are sometimes hardly 

 even hardened, and the limestones are not rendered in any appreciable degree more crystal- 

 line even up to the very margin of the intrusive rock. Where the igneous material has 

 been thrust between the strata in sheets, it has produced far more general and serious 

 metamorphism than when it occurs in the form of dykes. The famous rock of Portrush, 

 which was once gravely cited as an example of fossiliferous basalt, is a good illustration 

 of the way in which Lias shale is porcellanised when the intruded igneous material has 

 been thrust between its planes of bedding. 



In connection with the metamorphism superinduced by dykes, reference may be made 

 to the curious alteration which they themselves have sometimes undergone where they 

 have invaded a carbonaceous shale or coal. The igneous rock loses its dark colour and 

 obviously crystalline structure, and becomes the pale-yellow or white, dull, earthy substance 

 known to geologists as " white trap." The chemical changes involved in this alteration 

 have been described by Sir I. Lowthian Bell. # Dr Stecher has also discussed the 

 the alterations traceable by the aid of the microscope.t 



§ 16. Eelation of Dykes to the Geological Structure op the Districts 



which they traverse. 



In no respect do the Tertiary basic djdjes stand more distinguished from all the other 

 rocks of this country than in their extraordinary independence of geological structure. 

 The successive groups of Palaeozoic and Mesozoic strata follow each other in approximately 

 parallel bands, which run obliquely across the island from south-west to north-east. The 

 most important lines of fault take the same general line. The contemporaneously included 

 igneous rocks follow, of course, the trend of the stratified deposits among which they lie, 

 and even the intrusive eruptive rocks tend to group themselves along the general strike 

 of the whole country. But the Tertiary dykes have their own independent direction, to 

 which they adhere amid the extremest diversities of geological arrangement. 



* Proc. Roy. Soc, xxiii. (1875)*p. 543. 



t Tschermak's Mineralogische Mittheilungen, ix. (1887) p. 145; Proc. Boy. Soc. Edin., 1888. Dr Stecher's 

 investigation is based upon a series of specimens from the intrusive (Carboniferous) rocks of the basin of the Firth of 

 Forth, and has reference both to the phenomena of contact-metamorphism and the alteration of the eruptive rock ; 

 but these changes belong to the Carboniferous period. 



