286 NEW ZEALAND NATURE-STUDY BOOK 
X.—ROCKS. 
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INTRODUCTION.—A rock, we have seen, is a mass of matter 
composed of one or more simple minerals. In point of fact most * 
of the common rocks are made up of numerous crystalline or 
fragmentary particles which generally belong to several different 
kinds of minerals. : 
Classification. In attempting a classification of Rocks 
various plans may be adopted—we may arrange them 
according to their chemical composition, their general 
structure, or their mode of origin. A classification based 
on the two last considerations seems to offer greater 
chances of simplicity and altogether to be more suitable for 
our purpose. } 
If we examine a number of Rocks, we shall find for the 
most part that they can readily be divided into two main 
groups—viz. those whose particles are arranged in well- 
defined and regular layers, or strata, and those whose masses 
show no such arrangement. The former are known as 
stratified and the latter as unstratified Rocks. As strati- 
fication implies the agency of water, the former are some- 
times spoken of as aqueous or sedimentary Rocks, and 
as, moreover, the particles of which they are composed have 
(in many cases) been derived from the disintegration of 
older rock masses, they are also known as derivative 
Rocks. Sandstone, mud-rock, or shale will furnish examples 
of sedimentary or derivative Rocks. 
The unstratified Rocks are usually compact, crystalline, 
and of igneous origin—indeed the term unstratified is 
nearly coterminous with igneous. 
- Igneous Rocks are those which have been erupted from 
the heated interior of the earth, and include both volcanic 
rocks or those outflows of lava which have reached the 
surface, and plutonic or deep seated igneous masses which 
have cooled and consolidated at some distance from the 
