CHAPTER XXXV 
THE SYSTEM OB’ DYKES continued 
Direction — Termination upward — -Known vertical Extension — Evidence as to the move- 
ment of the Molten Rock in the Fissures — Branches and Veins — Connection of Dykes 
with Intrusive Sheets — Intersection of Dykes — Dykes of more than one infilling- — 
Contact Metamorphism of the Dykes — Relation of the Dykes to the Geological Struc- 
ture of the Districts which they traverse — Data for estimating the Geological Age of 
the Dykes — Origin and History of the Dykes. 
9. DIRECTION 
Another characteristic feature of the dykes is their generally rectilinear 
course. So true are the solitary dykes to their normal trend that, in spite 
of varying inequalities of surface and wide diversities of geological struc- 
ture in the districts which they traverse, they run over hill and dale almost 
with the straightness of lines of Roman road. In the districts where the 
gregarious type prevails, the dykes depart most widely from the character of 
the great solitary series, but still tend to run in straight or approximately 
straight lines, or, if wavy in their course, to preserve a general parallelism 
of direction. 
Yet even among the great persistent dykes instances may he cited where 
the rectilinear trend is exchanged for a succession of zig-zags, though the 
normal direction is on the whole maintained. In such cases, it is evident 
that the fissures were not long straight dislocations, like the larger lines of 
kiult in the earth’s crust, hut were rather notched rents or cracks which, 
though keeping, on the whole, one dominant direction, were continually being 
deflected for short distances to either side. As a good illustration of this 
character, reference may he made to the Cheviot and Hawick dyke. In 
Teviotdale, this dyke can he followed continuously among the rocky knolls, 
so that its deviations can be seen and mapped. From the median line of 
average trend the salient angles sometimes retire fully a quarter of a mile 
° n either side. Some examples of the same feature may he noticed in the 
Pskdale dyke. The large dyke which runs westward from Dunoon has been 
observed by Mr. Clough to change sharply in direction three times in four 
■vol. II l 
