44 
THE TERTIARY VOLCANOES 
BOOK VIII 
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 the strip of igneous rock 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 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 recognized is thus about 110 miles. But it probably goes 
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 dy ke 
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 prolonga- 
tion 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 sub- 
terranean 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 OP 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 
megascopic 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. 1 Similar though less^ pro- 
minent 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 gioundmass has been 
devitrified, and its former presence can only be detected with the aid ol the 
microscope. Where a dyke has caught up and absorbed abundant foreign 
materials its composition naturally varies considerably from point to point. 
Mr. Harker has observed some good examples oi this variation in Skye. 
1 See the careful examination of this dyke by Mr. Teall, Quart. Journ. Geol. Soc. xl. p. 209. 
