CHAP. XXXIV 
LENGTH OF THE D YKES 
143 
arises partly, no doubt, from the frequent spread of peat or other super- 
ficial accumulation which conceals the rocks, and partly also from the great 
number of dykes and the want of sufficiently distinct lithological characters 
for the identification ol any particular one. But making every allowance 
lor these obstacles, we are compelled, I think, to regard the gregarious dykes 
as essentially short as well as relatively irregular. 
In striking contrast to these, come the great solitary dykes. I 11 esti- 
mating their length, as I have already remarked, we must bear in mind the 
fact that they occasionally undergo interruptions of continuity owing to the 
local failure of the igneous material to rise to the level of what is now the 
surface of the ground. A narrow wall-like mass of andesite or dolerite, 
which sinks beneath the surface for a few hundred yards, or for several 
miles, and reappears on the same line with the same petrographical characters, 
while there may be no similar rock for miles to right and left, can only be 
one dyke prolonged underneath in the same great line of fissure. But even 
if we restrict our measurements of length to those dykes or parts of dykes 
where no serious interruption of continuity takes place, we cannot fail to be 
astonished at the persistence of these strips of igneous rock through the 
most diverse kinds of geological structure. A few illustrative examples of 
this feature may be selected. It will be observed that the longest and 
broadest dykes are found furthest from the basalt-plateaux, while the shortest 
and narrowest are most abundant near these plateaux. 
Not far from what I have taken provisionally as the northern boundary 
of the dyke region, two dykes occur which have been mapped from the head 
of Loch Goil by Arrocliar across Lochs Lomond and Katrine by Ben Ledi to 
Glen Artney, whence they strike into the Old lied Sandstone of Strathmore, 
and run on to the Tay near Perth — a total distance of about 60 miles. If 
the dyke which continues in the same line on the other side of the estuary 
of the Tay beyond Newburgh, is a prolongation of one of these, then its 
entire length exceeds 70 miles. A few miles further south, one of a group 
of dykes can be followed from the heart of Dumbartonshire by Callander 
across the Braes of Doune to Auchterarder — a distance of 47 miles, with an 
average breadth of more than 100 feet. In the district between the Forth 
and Clyde a number of long parallel dykes can be traced for many mil es 
across hill and plain, and through the coal-fields. One of these is continuous 
for 25 miles from the heart of Linlithgowshire into Lanarkshire. Still 
longer is the dyke which runs from the Firth of Forth at Grangemouth 
westward to the Clyde, opposite Greenock — a distance of about 36 miles. 
Coming southward, we encounter a striking series of single dykes on the 
uplands between the counties of Lanark and Ayr, whence they strike into 
the Silurian hills of the southern counties. One of these runs across the 
West of the Haughshaw Hills, and can be followed for some 30 miles. But 
as is probable, it is prolonged in one of the dykes that traverse the nioor- 
ands of the north of Ayrshire and south of Renfrewshire to the Clyde, its 
actual length must be at least twice that distance. The great Moffat and 
^skdale dyke strikes for more than 50 miles across the South of Scotland 
