

CTA PA TR. 2c 
ERUPTIVE ROCKS: MODE OF THEIR OCCURRENCE 
Intrusive Eruptive Rocks. Plutonic or Abyssal and Hypabyssal Rocks 
—their General Petrographical Characters. Batholiths—Granite as 
a type; phenomena along line of Junction with Contiguous Rocks ; 
Xenoliths ; speculations as to Assimilation of Rocks by Granite, etc. 
Laccoliths of North America. Sills or Intrusive Sheets appear to 
be much-denuded Laccoliths. Necks or Pipes of Eruption—their 
General Phenomena. 
IGNEOUS rocks have either been extruded at the surface, 
as in the case of volcanic eruptions, or they have cooled and 
consolidated below ground, and are now exposed to the light 
of day owing to the removal by denudation of the rock-masses 
underneath which they formerly lay concealed. We have 
thus two types of eruptive rocks, namely, effusive and intrusive, 
the latter of which is most conveniently described first. 
INTRUSIVE ..ERUPTIVE ROCKS 
These rocks are sometimes termed subsequent, with 
reference to the fact that they are of subsequent origin to 
the rock-masses with which they are associated. Two groups 
of intrusive rocks are recognised, namely, Plutonic or 
Abyssal and Hypabyssal rocks, the former having consoli- 
dated at great depths in the crust, while the latter are of 
less deep-seated origin. It must be admitted, however, that 
no clear line of demarcation separates these two groups— 
the one type of rock passing into the other. Nevertheless, 
the extremes of the two series are more or less strongly 
differentiated by their petrographical characters, and also to 
some extent by the mode of their occurrence. 
The Plutonic or more deeply seated rocks are. never 
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