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placement have been directly observed in many parts of 
the world. They are fine-grained, the crystals usually 
being far too small to be resolved with the naked eye. 
The voleanies are typically extruded on the surface and 
cooling is consequently rapid, with the result that 
crystal growth is inhibited. Sometimes cooling is too 
rapid for crystallisation to be completed and the rock 
may contain a certain amount of natural glass, and 
occasionally volcanic masses exhibit a chilled border 
consisting entirely of glassy material. 
The silica content of an igneous rock gives a useful 
indication of its overall chemical composition and a 
working classification . on this basis is frequently used 
when referring to these rocks. Silica enters into the 
composition of most rock-forming minerals and may also 
be present in tin 1 free state as Quartz. Those with a high 
silica content are said to be acid, and those with a low 
silica content arc said to he basic and these two groups 
grade one into the other through intermediate types. 
As a general ride the darker the colour, the more basic 
is the rock. Among the voleanies Rhyolites, which are 
acid, are white or pale pink or pale grey; Trachytes and 
Andesites, which are intermediate, range from grey to 
green or darker grey; and Basalts, which are basic, are 
almost black. The volcanic glasses, irrespective of their 
composition, are always dark in colour. 
There is a tendency to think of volcanic activity 
always in terms of violent eruptions, and Vesuvius and 
ICrakatoa are names that everybody knows. Volcanic 
activity frequently has been and is of a violent nature 
hut many of the large sheets of basalt, and basalt is the 
most common volcanic rock, have had their origin in quiet 
fissure eruptions, in which the molten lava has merely 
welled up at points along a fissure in the crust. Erup- 
tions of this kind have occurred in Iceland in modern 
times, and many of the basalts of S.E. Queensland prob- 
ably had such an origin. The sites of violent eruptions 
present vn Iranologists with many interesting topographic 
and geological structures. The cones and their craters, 
the disrupted cones or calderas and t lie volcanic plugs 
are often impressive features of the landscape. These 
plugs are the residual lava infillings of the vents which 
have been revealed by erosion. The Glasshouses and 
Peak Mountains in S.E. Queensland are examples of 
