Igneous Eocks. 
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flowed from the craters of some long-extinct volcanoes. 
The dykes intersecting the Hawkesbury Sandstone 
in so many directions are no less certainly rocks of 
Igneous origin. 
For convenience in their study, Igneous rocks 
must be classified. Geologists are not, however, 
agreed as to the best system of classification, although 
all are agreed on the characters which distinguish 
them. Igneous rocks are known — 
1. By occurring in irregular masses, penetrat- 
ing stratified rocks, and by sending off 
tongues and dykes into the sedimentary 
rocks around them. 
2. By their showing* characters not unlike 
furnace slags. 
8. By their vesicular structure. 
4. By their completely crystalline or partly 
crystalline structure. 
5. By their being identical with rocks now 
flowing from volcanic vents. 
G. By their altering the rocks they are in con- 
tact with. Sandstones, for instance, are 
altered to quartzite, and coal to a natural 
coke. 
Igneous rocks may be grouped according to — 
(a) Structure. 
( b ) Chemical composition. 
(c) Origin, 
