


= = J - - S tesa 
. P 
ory 
ae 
554 GHOTECTONIC (STRUCTURAL) GEOLOGY. {Book 
= 
may be seen projecting above ground exactly like walls (Fig. 281) ; 
indeed in many parts of the west of Scotland they are made use of 
for enclosures. ‘The material of the dykes has in other cases decayed, — 
and deep ditch-like hollows are left to mark their sites. The coast- 

ONE MILE 
S WOK 
WON AN 
\\\ \ 


a 
Fic. 280.—Map oF PART OF THE Mininc District or GwENnNAP, CORNWALL (B.). 
a a, Granite; ¢ c, Schistose rocks; b b, Elvan dykes; s, “ Greenstone,” v v, d d, two 
intersecting series of mineral veins. 
lines of many of the Inner Hebrides and of the Clyde Islands furnish 
numerous admirable examples of both kinds of scenery. 
The term dyke might be applied to some of the wall-like 
intrusions of quartz-porphyry, elvanite, and even of granite, but it is 
more typically illustrated among the augitic igneous rocks, such as 
basalt, diabase, &c., though also among diorites, porphyries, pitch- 
stones, &c., while occasionally dykes may be observed even of tuff 
and volcanic agglomerate. While veins have been injected into 
irregular branching cracks, dykes have been formed by the welling 
upwards of liquid or plastic rock in vertical or steeply inclined 
fissures, though obviously there is no essential difference between 
the two forms of structure. Sometimes the line of escape has been 
along a fault. In Scotland, however, which may be regarded as a 
typical region for this kind of geological structure, the vast majority 
of dykes rise along fissures which have no throw, and are therefore 
not faults. On the contrary, the dykes may be traced undeflected 
across some of the largest faults in the midland counties. 
‘7 ie ¥ 
hee ae . 
-Vo™ . 
‘ 
F 
zi 
