CHAPTER IV 
THE IGNEOUS OR ERUPTIVE ROCKS 
Definition of a Rock. — A rock is an accumulation 
of minerals forming a part of the earth’s crust. 
Usually several minerals are thus combined, though 
in some cases one alone forms the rock. We ordina- 
rily think of a rock as something hard and durable; 
but this is not necessarily so, for in many cases there 
is no line that can be drawn between the loose mineral 
fragments, and the solid rock. A great many of the 
strata of the earth have been solidified from the loose 
and friable clays or sand; and in many places this 
solidification is in progress at the present time (Fig. 12). 
On the Florida coast, in the Bermudas, and indeed 
commonly where lime-secreting animals thrive, the 
coral fragments that have recently been thrown above 
the sea-level by the waves, are cemented into a rock 
that can be used for building; and in such places the 
houses are constructed of this coguina (Fig. 30). Also, 
in many gravel banks, there are hard cemented layers, 
side by side with sand and gravel, that men are carting 
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