DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 49 



length, as I have already remarked, we must bear in mind the fact that they occasionally 

 undergo interruptions of continuity owing to the local failure of the igneous material to 

 rise to the level of what is now the surface of the ground. A narrow wall-like mass of 

 augite-andesite, dolerite, or basalt which sinks beneath the surface for a few hundred 

 yards, or for several miles, and reappears on the same line with the same petrographical 

 characters, while there may be no similar rock for miles to right and left, can only be one 

 dyke prolonged underneath in the same great line of fissure. But even if we restrict our 

 measurements of length to those dykes or parts of dykes where no serious interruption of 

 continuity takes place, we cannot fail to be astonished at the persistence of these strips 

 of igneous rock through the most diverse kinds of geological structure. A few illustrative 

 examples of this feature may be selected. It will be observed that the longest and 

 broadest dykes are found furthest from the basalt-plateaux, while the shortest and 

 narrowest are most abundant near these plateaux. 



Not far from what I have taken provisionally as the northern boundary of the dyke 

 region, two dykes occur which have been mapped from the head of Loch Goil by 

 Arrochar across Lochs Lomond and Katrine by Ben Ledi to Glen Artney, whence they 

 strike into the Old Eed Sandstone of Strathmore, and run on to the Tay near Perth— a 

 total distance of about 60 miles. If the dyke which continues in the same line on the 

 other side of the estuary of the Tay beyond Newburgh, is a prolongation of one of these, 

 then its entire length exceeds 70 miles. A few miles further south, one of a group of 

 dykes can be followed from the heart of Dumbartonshire by Callander across the Braes of 

 Doune to Auchterarder — a distance of 47 miles, with an average breadth of more than 

 100 feet. In the district between the Forth and Clyde a number of long parallel dykes 

 can be traced for many miles across hill and plain, and through the coal-fields. One of 

 these is continuous for 25 miles from the heart of Linlithgowshire into Lanarkshire. 

 Still longer is the dyke which runs from the Firth of Forth at Grangemouth westward 

 to the Clyde, opposite Greenock — a distance of about 36 miles. Coming southward, we 

 encounter a striking series of single dykes on the uplands between the counties of 

 Lanark and Ayr, whence they strike into the Silurian hills of the southern counties. 

 One of these runs across the crest of the Haughshaw Hills, and can be followed for some 

 30 miles. But if, as is probable, it is prolonged in one of the dykes that traverse the 

 moorlands of the north of Ayrshire and south of Renfrewshire to the Clyde, its actual 

 length must be at least twice that distance. The great Moffat and Eskdale dyke strikes 

 for more than 50 miles across the south of Scotland and north of England. The Hawick 

 and Cheviot dyke runs for 26 miles in Scotland and for 32 miles in Northumberland. 

 But the most remarkable instance of persistence is furnished by the Cleveland dyke. 

 From where it is first seen near the coast-cliffs of Yorkshire it can be followed, with frequent 

 interruptions, during which for sometimes several miles no trace of it appears at the 

 surface, across the north of England and as far as Dalston Hall south of Carlisle, beyond 

 which the ground onwards to the Solway Firth is deeply covered with superficial deposits. 

 The total distance through which this dyke can be recognised is about 110 miles. But 



