DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



61 



A microscopical examination of the specimens likewise shows that they are perfectly 

 identical in composition and structure, chiefly referable to rocks of the dolerite type, but 

 partly to the tholeiite type. I have therefore no doubt that these remarkable 

 appendages to this dyke are truly offshoots from it, and are not to be classed with the 

 general mass of the diabases of central Scotland, which are Lower Carboniferous. The 

 accompanying diagrammatic section (fig. 15) explains what appears to me to be the 

 structure of the ground. 



An interesting and important fact remains to be stated in connection with these 

 sheets. They are traversed by some of the other east and west dykes. This is particu- 

 larly observable in the case of the sheet which extends northwards from the dyke through 



Fig. 15. 



-Section to show the connection of a Dyke with an Intrusive Sheet, Stirlingshire Coal-field, a, Dyke in line of fault : 

 b, intrusive sheet traversing and altering the coals ; i, Slaty-band Ironstone. 



the parish of Torphichen. Two well-marked dykes can be seen running westwards 

 among the ridges of the sheet. It is obvious, therefore, that these particular dykes are 

 younger than the sheet. But, as will be shown in the sequel, there is abundant evidence 

 that all the dykes of a district are not of one eruption. The intersection of one eruptive 

 mass by another does not necessarily imply any long interval of time between them. 

 They mark successive, but it may be rapidly successive, manifestations of volcanic action. 

 Hence the cutting of the sheets by other dykes does not seem to me to invalidate the 

 identification of these sheets as extravasations from the great dyke by which they are 

 bounded. 



§ 14. Intersections of Dykes — Repeated Dykes in the same Line of Fissure. 



Innumerable instances may becited, where one dyke or one set of dykes cuts across 

 another. To some of these I shall refer in discussing the data for estimating the relative 

 age of dykes. In considering the intersection from the point of view of geological 

 structure, we are struck with the clean sharp way in which it so generally takes place. 

 The rents into which the younger dykes have been injected seem, as a rule, not to have 

 been sensibly influenced in width or direction by the older dykes, but go right across 

 them. Hence the younger dykes retain their usual breadth and trend (fig. 16). The 

 most interesting examples are those in which one dyke runs along another, as may 

 occasionally be observed in the west of Scotland."" In these cases, which are to be 

 distinguished from those where the whole may be really a portion of one original slowly 

 cooling mass, the central dyke differs sufficiently in texture and structure to be discrimin- 



* Macculloch figured an example from Strathaird, Western Islands, pi. xviii. fig. 1. Mr Clough has found some 

 good instances in south-eastern Argyleshire. 



VOL. XXXV. PART 2. I 



