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DESCRIPTIVE CATALOGUE, 

IGNEOUS ROCKS. 

PLUTONIC AND VOLCANIC. 
The relations of these rocks indicate that they have, for the most 
part, been forced upwards from the interior of the earth in a fluid or 
semi-fluid (viscous) state. They have penetrated and overflowed other 
formations, and then become solid under varying cdhditions, either on 
or beneath the surface. ‘The soft state in which they existed was 
probably due to great heat—hence the term ‘Igneous rocks ;” and their 
texture, as we now find them, is the result, partly, of the particular 
conditions under which they became solid, and is partly due to subse- 
quent metamorphism. ‘They are either compact, porphyritic, or crys- 
talline granular, seldom fissile, but frequently vesicular or amygdaloidal. 
They also often occur in a wackenitic state (wacke*), this form being, in 
Victoria, very characteristic of the rocks of the Older Volcanic series. As 
volcanic ash, they occur in stratified layers, having much the appearance 
of true sedimentary aqueous rocks. ‘They are classed under two principal 
divisions, expressive of their origin. , 
1. PLuroxic—or those that became solid at a considerable depth 
in the interior of the earth (consequently under great pres- 
sure), and have been exposed at the surface by the removal 
of the once superincumbent strata. 
2. Votcanic—or those that have been forced to the surface, and 
either flowed over it in a molten state, or been spread over 
it as ash. i 
These again may be divided into two classes expressive of their 
constitution :— 
1. Acıpıc—or those rich in silica. 
2. Basıc—or those poor in silica, 
The average composition of these two classes of igneous rocks is, 
according to Cotta— 
‘ 
Basic rocks, Acidic rocks, 
Silica we ... 45—55 60—80 
Alumina «. 10—20 8—16 
eae } Tron. sss 1-15 115 
Lime an AS 1—10 1— 5 
Magnesia ... -- I—6 0— 4 
Potash or .. l—4 1— 6 
Soda y aw = =l— 5d 1— 6 
Nater = ... ee tO — AL. 0— 8 

* Wacke is a German miner’s term used to describe a decomposed state of 
igneous rocks poor in silica. 
B 
