DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 33 



Of this extensive region the greater portion has now been mapped in detail by the 

 Geological Survey. Every known dyke has been traced, and the appearances it 

 presents at the surface have been recorded. We are accordingly now in possession 

 of a larger body of evidence than has ever before been available for the discussion 

 of this remarkable feature in the geology of the British Isles. I have made use of 

 this detailed information, and besides the data accumulated in my own note-books, I have 

 availed myself of those of my colleagues in the Survey, for which due acknowledgment 

 is made where they are cited. 



§ 2. Two Types of Protrusion. 



The dykes are far from being equally distributed over the wide region within 

 which they occur. In certain limited areas they are crowded together, a score or more 

 occurring in a single square mile, while elsewhere they appear only at intervals of several 

 miles. Viewed in a broad way, they may be conveniently grouped in two types, which, 

 though no hard line can be drawn between them, nevertheless probably point to two more 

 or less distinct phases of volcanic action. In the first, which for the sake of distinction 

 we may term the Solitary type, there is either a single dyke separated from its nearest 

 neighbours by miles of intervening and entirely dykeless ground, or a group of two 

 or more running parallel to each other, but sometimes a mile or more apart. The rock 

 of which they consist is, on the whole, less basic than in the second type ; it includes 

 the andesitic varieties. It is to this type that the great dykes of the north of England 

 and the south and centre of Scotland belong. The Cleveland dyke, for example, at its 

 eastern end has no known dyke near it for many miles. The coal-field of Scotland is 

 traversed by five main dykes, which run in a general sense parallel to each other, with 

 intervals of from half a mile to nearly five miles between them. Dykes of this type 

 display most conspicuously the essential characters of the dyke-structure, in particular 

 the vertical marginal walls, the parallelism of their sides, their great length, and their 

 persistence in the same line. 



In the second, or what for brevity may be called the Gregarious type, the dykes 

 occur in great abundance within a particular district. They are on the whole 

 narrower, shorter, less strikingly rectilinear, more frequently tortuous and vein-like, and, 

 on the whole, more basic in composition than those of the first type. They include the 

 true basalts and dolerites. Illustrative districts for dykes of this class are the islands of 

 Arran, Mull, Eigg, and Skye. 



The great single or solitary dykes may be observed to increase in number, though 

 very irregularly, from south to north, and also in central Scotland from east to west. 

 They are specially abundant in the tract from the Firth of Clyde along a belt of country 

 some thirty miles broad on either side of the Highland line, as far at least as the valley 

 of the Tay. They form also a prominent feature in the islands of Jura and Islay. 

 Those of the gregarious type are abundantly and characteristically displayed in the 

 basin of the Firth of Clyde. Their development in Arran formed the subject of an 



