OF THE TRAP-ROCKS OF SCOTLAND. 651 
Beginning at the south-east side of Scotland and the northern part of Eng- 
land, we find long dykes of greenstone traversing the country in rectilinear lines. 
One of these was described by Mr Mitne-Homn* in his Memoir on the Geology 
of Roxburghshire. It runs in a west north-west direction from beyond Hawick 
to the crest of the Cheviot Hills near Hyndhope, whence it runs eastward to the 
sea. Another dyke, with a nearly similar direction, was observed by myself in 
the north part of Northumberland. One branch crosses the Tweed below Cold- 
stream and meets another a few miles toward the east, whence both range 
south-eastward to Holy Island. If the line of the Hawick dyke be prolonged 
towards the north-west, it will be found to pass through the small coal-field 
of Douglas. It is remarkable that in that coal-field, passing north-westwards 
across the Haughshaw Hills, a large greenstone dyke does occur, the direc- 
tion of which I found to be similar to that of the Hawick one. In the same 
district also I detected another massive dyke running parallel to the last, and 
crossing the upper part of the valley of Muirkirk. These dykes cross all the 
other igneous rocks of the district in which they occur, and are plainly the latest 
formed. 
In the Lothians, the larger dykes have an east and west, or west-by-north-east 
by-south direction, sometimes, however, veering to west-by-south, east-by-north. 
Their line of strike appears to be, in most cases, determined by some previous 
line of fissure, and their course can usually be determined with ease even where 
they cut through masses of greenstone of similar lithological characters. Some 
instructive examples of this kind occur in the hilly ground south of Linlithgow. 
In Bute and Cantyre a north-westerly trend characterises the greenstone 
dykes, and the same direction is maintained in Islay and Jura.t From these 
islands northward, along the whole of the wild western coast, nothing can be 
more striking than the persistence of this north-westerly trend. Whether the 
rock traversed be crystalline syenite, metamorphic gneiss, contorted schist, or 
gently-inclined oolitic sandstones and shales, the dykes of greenstone are seldom 
diverted from their course. They run across lonely glens and up the sides of 
rugged mountains like lines of ruined ramparts, and then away out to sea like 
huge moles and breakwaters raised by some superhuman agency against the 
fury of the western waves. In some localities, as, for example, in the little island 
of Pabba, between Skye and the mainland, they occur in such numbers that one 
might almost cross the district by stepping from dyke to dyke.t 
* On the Geology of Roxburghshire.—Trans. Roy. Soc. Edinb., vol. xv. p. 456. 
} In these islands there is an older group of magnesian greenstones, which run parallel to the 
strike of the schists—+.¢., from south-west to north-east. It is instructive to see how completely 
these are cut through by the newer group, I have described these features in a joint paper on the 
Metamorphic Rocks of the Highlands, by Sir RopErick Murcuison and myself (Quart. Jour. Geol. 
Soc., vol. xvii. p. 210 ). The same facts have been observed by Mr Jamzson in Cantyre and Knap- 
_ dale (Quart. Jour. Geol. Soc., vol. xvii. p. 140). 
t See my paper on Skye and Pabba (Quart. Jour. Geol. Soc., vol. xiv. p. 1, et seq). 
