CHAP. XXXV 
GEOLOGICAL AGE OF THE DYKES 
17 
Glen Artney. They then turn round towards south-west, and run up the 
glen along the strike of the beds, keeping approximately parallel to the fault 
for about three miles, when they both strike across the fault, and pursue a 
T7.S.W. line through the contorted crystalline rocks of the Highlands. 
About two miles further south, another dyke continues its normal course 
across the belt of upturned Old Bed Sandstone ; but when it reaches the 
fault it bends round and follows the line of dislocation, sometimes coinciding 
with, sometimes crossing or running parallel with that line, at a short 
distance (see Fig. 247). 
Some remarkable examples have been mapped by Mr. Clough in 
Eastern Argyleshire, where broad bands of basalt or other allied rock run in 
a hi. and S. direction, and are formed by the confluence of NAV and S.E. or 
N'.N’.W. and S.S.E. dykes, where these are drawn into a line of fault (Fig. 
257). These broad bands, he has found to be not usually traceable for 
more than a mile or so, for the dykes of which they are made up will not 
be diverted from their regular paths for more than a certain distance, so 
that one by one the dykes leave the compound band to pursue their normal 
course. He has observed that the occasional great thickness of these com- 
pound bands depends partly on the size and partly on the number of 
separate dykes that are diverted into the line of transverse fissure ; for, 
where the fissure crosses an area with fewer north-west dykes, the band 
becomes thinner or ceases altogether. 
In some rare cases, the dykes have been shifted by more recent faults. 
I shall have occasion to show that faults of more than 1000 feet have taken 
place since the Tertiary basalt-plateaux were formed. There is therefore no 
reason why here and there a fault with a low hade should not have shifted 
the outcrop of a dyke. But the fact remains, that, as a general rule, the 
dykes run independently of faults even where they approach close to them. 
Mr. Clough has observed some interesting cases in South-eastern Argyleshire, 
where the apparent shifting of a dyke by faults proves to be deceptive, and 
where the dyke has for short distances merely availed itself of old lines of 
fracture. One of the most remarkable of these is presented by the large 
dyke which runs westward from Dunoon. No fewer than three times, in 
the course of four miles between Lochs Striven and Eiddon, does this dyke 
make sharp changes of trend nearly at right angles to its usual direction, 
where it encounters north and south faults (Fig. 257). It would be natural to 
conclude that these changes are actual dislocations due to the faults. But 
the careful observer just cited has been able to trace the dyke in a very 
attenuated and uncrushed form along some of these cross faults, and thus to 
prove that the faults are of older date, but that they have modified the line 
of the long east and west fissure up which the material of the dyke ascended. 
19. DATA FOR ESTIMATING THE GEOLOGICAL AGE OF THE DVHES 
I have already assigned reasons for regarding the system of north-west 
and south-east or east and west dykes as belonging to the Tertiary volcanic 
