582 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. — 
they look like beds. At these parts, they are of course really — 
intrusive sheets. But they may frequently be found to start 
suddenly upward or downward, and to break across the bedding 
in a very irregular manner. ‘ 
- 
No rock exhibits more instructively than granite the numerous 
varieties of form assumed by veins. One large class of granite veins is 
probably referable to segregation-veins—indeed in the case of those asso- 
ciated with granitoid gneiss, it seems impossible to draw any line between 
segregation and eruption. Where veins proceeding from a granite 
mass traverse disrupted strata of schist or gneiss, they may be intrusive, 
though this by no means always follows; for in the archean gneiss 
of Sutherland the abundant pegmatite veins, even when cutting across 
disrupted bands of gneiss, pass into others that are interbedded with 
and graduate insensibly into the gneiss, so that the whole mass, veins 
and folia alike, must be regarded as due to the same great complex 
process—that which produced the ancient crystalline schists. Most 
large masses of granite send veins into the surrounding rocks, and often 
in such abundance as to form a complicated network (Figs. 278, 279). 







Fic, 278.—GRANITE VEINS. 
They vary in breadth from several feet or even yards down to fine fila- 
ments at the ends of the smaller branches. They frequently cross each 
other, not only outside of the granite mass, but even within it. They 
vary much in texture and in composition. Sometimes they are coarsely 
crystalline pegmatite, but most of the veins of this kind are doubtless due 
rather to segregation than intrusion. Large bosses of granite are often 
traversed by conspicuous veins of pegmatite (Fig. 284), but the veins due 
most probably to actual intrusion of material, are commonly finer-grained 
than the main mass. Besides this greater closeness of texture, these 
intrusive veins sometimes present considerable differences in mineralogical 
composition. The mica, for example, may be reduced to exceedingly 
minute and not very abundant flakes, and may almost disappear. The 
quartz also occasionally assumes a subordinate place, and the rock of 
the veins passes into eurite, elvanite, or one of the varieties of felsite or 
quartz-porphyry.' 
Where granite appears among crystalline schists, the distinctive 
characters of its intrusive veins are apt to be lost among the abundant 
' See a reference to the Bodegang, ante, p. 1384. Mr. Hawes has recently described 
a similar example from New Hampshire. Amer. Journ. Sct, xxi. (1881), p. 244. 

