50 DR UE'lKIE ON THE HISTORY OF VOLCANIC ACTION 



it probably goes much further still. On the opposite side of the Solway, a dyke which 

 runs in the same line, rises through the Permian strata a little to the east of the 

 mouth of the Nith. Some miles further to the north-west, near Moniaive, Mr J. Horne, 

 in the progress of the Geological Survey, traced a dark compact dyke with kernels of 

 basalt-glass near its margin, running in the same north-westerly direction. Still further 

 on in the same line, another similar rock is found high on the flanks of the lofty hill 

 known as Windy Standard. And lastly, in the Ayrshire coal-field, a dyke still continuing 

 the same trend, runs for several miles, and strikes out to sea near Prestwick. It cannot, 

 of course, be proved that these detached Scottish protrusions belong to one great dyke, or 

 that if such a continuous dyke exists, it is a prolongation of that from Cleveland. At 

 the same time, I am on the whole inclined to connect the various outcrops together as 

 those of one prolonged subterranean wall of igneous rock. The distance from the last 

 visible portion of the Cleveland dyke near Carlisle to the dyke that runs out into the 

 Firth of Clyde near Prestwick, is about 80 miles. If we consider this extension as a 

 part of the great north of England dyke, then the total length of this remarkable 

 geological feature will be about 190 miles. 



§ 8. Persistence of Mineral Characters. 



Not less remarkable than their length is the preservation of their normal petrographical 

 characters by some dykes for long distances. In this respect the Cleveland dyke may 

 again be cited as a typical example. The macroscopic and microscopic structures of 

 the rock of this dyke distinguish it among the other eruptive rocks of the north of 

 England. And these peculiarities it maintains throughout its course.* Similar though 

 less prominent uniformity may be traced among the long solitary dykes of the south of 

 Scotland, the chief variations in these arising from the greater or less extent to which 

 the original glassy magma has been retained. The same dyke will at one part of its 

 course show abundant glassy matter even to the naked eye, while at a short distance the 

 vitreous ground-mass has been devitrified, and its former presence can only be detected 

 with the aid of the microscope. 



§ 9. Direction. 



Another characteristic feature of the dykes is their generally rectilinear course. 

 So true are they to their normal trend that, in spite of varying inequalities of surface 

 and wide diversities of geological structure in the districts which they traverse, they 

 run over hill and dale almost with the straightness of lines of Eoman road. In the 

 districts where they assume the gregarious type, and depart most widely from the character 

 of the great solitary dykes, they still tend to run in straight or approximately straight 

 lines, or, if wavy in their course, to preserve a general parallelism of direction. 



Yet even among the great persistent dykes instances may be cited where the recti- 

 linear trend is exchanged for a succession of zig-zags, though the normal direction is 



* See the careful examination of this dyke by Mr Teall, Quart. Jour. Geol. Soc, xl. p. 209. 



