

STRUCTURE OF ERUPTIVE ROCKS 207 
have forced their way along planes of cleavage or of foliation. Asa rule, 
however, small and large veins alike follow no definite direction—save 
that they stream outwards from the margin of the parent batholith— 
gradually diminishing in numbers as they proceed. In many cases the 
veins form a perfect network amongst which irregular fragments and 
larger masses of the invaded rocks appear as if entangled—forming what 
is termed an zmjection plexus. All the phenomena, indeed, seem to sug- 
gest that before the veins were injected the rocks surrounding a batholith 
had been so profoundly shattered, that molten matter found little diffi- 
culty in making its way amongst the fractured and sundered masses 
(Pig. 77).* 
The rock of these veins, especially the smaller ones, is usually finer 
grained than the granite-mass from which it comes. It is notable, also, 
that not infrequently it differs in petrographical character from that of 
the parent-rock—many of the veins consisting of quartz-porphyry or 
felsite. 
Endogenous or Autogenous Veins.—Some of these are composed of 
finer grained rock than the granite, and usually differ from it in being 
more acid. Others, again, are characterised by the intergrowth of the 
constituent quartz and felspar. These are the pegmatite-veins. They 
are generally coarser grained than the rock they traverse. The precise 
mode of origin of these endogenous veins is quite uncertain. Although 
obviously younger than the rock they cut, they yet appear to form portions 
of the same intrusive mass—to be merely modifications, as it were, of the 
granite itself. Hence they are often spoken of as contemporaneous veins. 
They are supposed to belong to the period of cooling and consolidation, and 
to have been injected from still liquid portions of the magma into rents 
formed during movements of the surrounding solidified or partially solidified 
mass. ‘This seems a plausible explanation of the fine-grained autogenous 
veins, but it does not account for the structure of the coarsely crystalline 
pegmatite veins.t The contemporaneous origin of both fine-grained and 

* The exploitation of “contact ore-formations” (see Chapter XVII.) 
has shown that the ore-bearing rocks overlying and surrounding a 
plutonic mass are often much jumbled and shattered—shales and lime- 
stones, for example, being converted into breccias which are usually 
highly silicified. These brecciated masses may occur at a considerable 
distance from the intrusive rock, and possibly owe their origin to the 
explosive action of steam and vapours. Not infrequently they are 
traversed by dykes and eruptive veins, but these could not have caused 
the shattering of the rocks, for the same dykes cut through undisturbed 
areas where no brecciation is visible. 
t+ According to Professor Arrhenius, a granite magma containing 
sufficient water would, in cooling, probably separate into two portions—the 
product of the separation appearing as an aqueous solution in which would 
be concentrated such bodies as are more soluble in water than in the silicate 
magma. Owing to their greater mobility than the magma, these aqueous 
