
tae A a | 
206 STRUCTURAL AND FIELD GEOLOGY 
central quartz-gabbro. Phenomena of this kind are doubt- 
less due to magmatic differentiation. 
Eruptive veins and dykes, as already indicated, often 
follow somewhat erratic courses. The more or less regular 

Fic. 76.—COMPOSITE DYKE, LIEBENSTEIN (THURINGIA), (After Dr 
K. Keilhack). 
a, a, granite; m, m, melaphyre; s, s, syenite-porphyry; G-p, granite-porphyry. 
basalt-dykes of Central Scotland have been cited as good 
examples of wall-like intrusions. It need hardly be said, 
however, that injections of basalt, as of any other kind of 
igneous rock, are often extremely tortuous and branching (see 
Plate XLVI.). The veins usually associated with granite, 
however, may be taken as somewhat characteristic of their 
kind. Of these veins two types are recognised—exrogenous or 
intrusive and autogenous or endogenous Veins. 
y PY , (24 We 
UG je ; \ Ni 
yee anes = soy " 
iit / ta & D\\i me < i 
Oo om » . v Ui Gs | 
7/35 
sia DP 
>" Yow Wa 

FIG. 77..-VEINS PROCEEDING FROM A MASS OF GRANITE, 
Exogenous or Intrusive Veins.—These are simply protrusions pro- 
ceeding from a mass of granite into the contiguous rocks, They vary in 
thickness from mere lines or threads up to many feet or yards. Usually 
very tortuous, they ramify in all directions, intercrossing, dividing and 
subdividing again and again. Now and again, extremely thin veins 

