aggregate horizontal extent of all these dykes, or of the fissures
which they fill, must be very considerable, it is clear that the crust
through which they have been extruded has received an accession of
horizontal space, and has been fissured by forces acting from beneath,
as the late Mr. Hopkins, of Cambridge, had explained on mechanical
grounds in his elaborate essay many years ago.[10] This view occurred to
myself when examining the region of the North-east of Ireland, but I was
not then aware that it had been dealt with on mathematical principles by
so eminent a mathematician. The bulging of the crust is a necessary
consequence of the absence of plication of the strata due to the
extrusion of this enormous quantity of molten lava; and the intrusion of
thousands of dykes over the North-east of Ireland, unaccompanied by
foldings of the strata, must have added a horizontal space of several
thousand feet to that region.[11]

[1] A peculiar form of crystalline quartz first recognized in this rock
by a distinguished German petrologist, the late Prof. A. von Lasaulx,
who visited the district in 1876.

[2] Sir A. Geikie has disputed the correctness of the view, which I
advocated as far back as 1874, that the trachytic lavas of Antrim are
the earliest products of volcanic action; but at the time he wrote his
paper on the volcanic history of these islands, it was not known that
pebbles of this trachyte are largely distributed amongst the ash-beds
which occur in the very midst of the overlying basaltic sheets, as I
shall have to explain later on. This discovery puts the question at rest
as regards the relations of the two sets of rocks.

[3] This remarkable section at the chalk quarries of Templepatrick the
author has figured and described in the _Physical Geology and Geography
of Ireland_, p. 99, 2nd edit. (1891), where the reader will find the
subject discussed more fully than can be done here.

[4] These pebbles were first noticed by Mr. McHenry, of the Irish
Geological Survey, in 1890.

[5] The vertical position of the columns of the Giant's Causeway is
rather enigmatical. The Causeway cannot be a dyke, as has often been
supposed, otherwise the columns would have been horizontal, _i.e._, at
right angles to the sides of the dyke. Mr. R. G. Symes, of the
Geological Survey, has suggested that the Causeway columns have been
vertically lowered between two lines of fault, and that originally they
formed a portion of the