Igneous Rocks. 
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centage of silica they contain — Acid, Intermediate, 
and Basic. Ultra-basic rocks are sometimes placed 
in a separate class. 
CLASSIFICATION OF ERUPTIVE ROCKS, 
According to their Origin and Composition. 
ACIDIC ROCKS, 
65 to 80 per cent, of Silica. 
Volcanic. 
Plutonic. 
OBSIDIAN GRANITE 
RHYOLITE 
INTERMEDIATE ROCKS, 
55 to 65 per cent, of Silica. 
Yolcanic. 
Plutonic. 
TRACHYTE 
SYENITE 
ANDESITE 
DIORITE (mostly) 
BASIC ROCKS, 
45 to 55 per 
cent, of Silica. 
Volcanic. 
Plutonic. 
BASALT 
DIORITES (some) 
DOLERITE 
GABBRO 
TACHYLITE 
According to origin , Igneous rocks are termed 
Plutonic if they consolidated at great depths and 
under great pressure, in those regions assigned by 
mythology to Pluto. On the contrary, fire-formed 
