CHAPTER XXXIY 
THE SYSTEM OE DYKES IN THE TERTIARY VOLCANIC SERIES 
Geographical Distribution — Two Types of Protrusion — Nature of Component Rocks — 
Ha.de — Breadth — Interruptions of Lateral Continuity — Length — Persistence of 
Mineral Characters. 
If a geologist were asked to select that feature in the volcanic geology of 
the British Isles which, more than any other, marks this region off from the 
rest of the European area, lie would probably choose the remarkable system 
of wall-like masses of erupted igneous rock, to which the old Saxon word 
“ dykes ” has been affixed. Erom the moors of eastern Yorkshire to the 
Perthshire Highlands, and from the basins of the Forth and Tay to the west 
of Donegal and the far headlands of the Hebrides, the country is ribbed 
across with these singular protrusions to such an extent that it may be 
regarded as a typical region for the study of the phenomena of dykes. 
That all the dykes in this wide tract of country are of Tertiary age cannot 
be maintained. It has been shown in previous Chapters that each of the 
great volcanic periods has had its system of dykes, even as far back as the 
time of the Lewisian Gneiss. 
But when all the dykes which can reasonably be referred to older 
geological periods are excluded, there remains a large series which cannot 
be so referred, but which are connected together by various kinds of 
evidence into one great system that must be of late geological date, and 
can be assigned to no other than the Tertiary period in the volcanic history 
of Britain. As far back as the year 1861, when I first drew attention to 
this great system of dykes in connection with the progress of volcanic 
action in the country, I pointed out the grounds on which it seemed to me 
that these rocks belong to a comparatively recent geological period. 1 My 
own subsequent experience and the full details of structure collected by 
my colleagues of the Geological Survey in all parts of the country, have 
amply confirmed this view. The characters which link this great series ol 
dykes together as one connected system of late geological date are briefly 
enumerated in the following list, and will be more fully discussed in later 
pages. 
1. The prevalent tendency of the dykes to take a north-westerly 
course. There are exceptions to this normal trend, especially where the 
1 Trans. Roy. Soc. Edin. vol. xxii. (1861), p. 650. 
