
7 
Parr VIL. Sucr. i. §3.] GRANITE VEINS. 558 
proofs of segregation. But where a large boss rises in a region of 
ordinary sedimentary rocks, these characters are strongly defined. It is 
in the metamorphosed belt, already (p. 542) described as encircling an 
intrusive boss of granite, that eruptive veins are typically developed 
and most readily studied. In Cornwall, for example, the granite and 
surrounding slates are abundantly traversed by veins or dykes of 
granite and of quartz-porphyry (elvans), which are most numerous near 
the granite. They vary in width from a few inches or feet to 50 fathoms, 
their central portions being commonly more crystalline than the sides. 
They frequently enclose angular fragments of slate (p. 543, note). Inthe 
great granite region of Leinster Mr. Jukes traced some of the elvans for 
several miles running in parallel bands, each only a few feet thick, with 
intervals of 200 or 300 yards between them. Around some of the 
granite bosses of the south of Scotland similar veins of felsite and 

Fig. 279.—SEcTION OF GRANITE (a), SENDING 4 NETWORK OF VEINS INTO SuaTE (0), 
CoRNWALL (B.). 
porphyry abound. The granite of the Wahsatch Mountains in Utah, 
which rises through the Upper Carboniferous limestones, converting 
them into white marble, sends out veins of granite-porphyry and other 
crystalline compounds. In short, all over the world it is common for 
eruptive bosses of this rock to have a fringe of intrusive veins (Fig. 280). 
Many other eruptive rocks (diorite, diabase, melaphyre, basalt, &c.) 
present admirable examples of intrusive veins. These are distinguished 
from those of granite by the much less metamorphism with which they 
are attended. 
Dykes are veins of eruptive rock, filling vertical or highly- 
inclined fissures, and are sonamed on account of their resemblance 
to walls (Scotice, dykes). Their sides are often as parallel and 
perpendicular as those of built walls, the resemblance to human 
workmanship being heightened by the numerous joints which, 
intersecting each other along the face of a dyke, remind us of well- 
fitted masonry. Where the surrounding rock has decayed, the dykes 
