

CHAP TE Re chy 
ERUPTIVE ROCKS: MODE OF THEIR OCCURRENCE— 
continued 
Dykes and Eruptive Veins—their General Phenomena. Composite 
Dykes. Exogenous or Intrusive Veins—their association with 
Batholiths, etc. Endogenous or Autogenous Veins. Pegmatite 
Veins ; General Phenomena of Contemporaneous Veins. Segrega- 
tion Veins. Effusive Eruptive Rocks—Crystalline Effusive Rocks 
and Pyroclastic or Fragmental Effusive Rocks. 
4. DYKES AND ERUPTIVE VEINS 
ERUPTED matter which has solidified in a more or less 
steeply inclined or vertical and somewhat even-sided fissure, 
is called a dyke, while the term eruptive vein is usually 
reserved for the more irregular and frequently tortuous and 
branching intrusions. But this usage is not invariable— 
many geologists employing the terms interchangeably, while 
others designate as “dykes” all the larger intrusions, whether 
wall-like or tortuous, and restrict the term “vein” to the 
smaller injections. 
Eruptive veins and dykes may consist of almost any kind 
of igneous rock. Frequently they proceed visibly from large 
masses of eruptive rock—bosses or sheets, as the case may be. 
At other times no such relationship can be _ observed, 
although we can hardly doubt that if dykes and veins could 
be followed downwards they would be found to proceed in 
the same way from larger masses of intrusive character. 
Wall-like intrusions are of common occurrence in this 
country—the most notable examples being the remarkable 
basalt-dykes which are so abundantly developed in Central 
and Western Scotland (see Plate XLVIII.). Sometimes these 
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