ROCKS 287 
surface. All these Rocks are compact, more or less 
crystalline, and unstratified. 
Certain Rocks are met with which present a combination 
both* of stratified and unstratified rocks, 7.e. they possess 
crystalline structure in a greater or less degree, and though 
not arranged in regular layers, yet often show unmistake- 
able signs of stratification. They are in fact Rocks of 
whose sedimentary origin there can be little or no doubt ; 
but whose structure has undergone a considerable amount 
of alteration since their original formation. These are 
called metamorphic Rocks of which Slate, Schist, and 
_ some Granites are examples. 
Stratified or sedimentary Rocks must clearly have been 
derived from the disintegration of previously formed rock 
masses. Hence, in order to account for the formation of 
stratified Rocks, we must suppose the existence of masses 
from which their materials were derived. For this reason it 
is proposed to deal with unstratified or igneous Rocks 
first. 3 
For our purpose then we may classify rocks as : 
(1) Unstratified or igneous Rocks. 
(2) Stratified or sedimentary Rocks, including all which 
have been laid down as mechanical or chemical deposits 
from water or air, and all which have resulted from the 
growth and decay of plants and animals. 
(3) Metamorphic Rocks, or those which have undergone 
subsequent alteration within the crust of the earth, 
whereby their original characters have been changed. 
In order thoroughly to appreciate even this brief classi- 
fication, pupils should observe exposures of Rocks in the 
open country—such as are to be met with in pits, quarries, 
and cuttings of all kinds. It is only by examinations of 
this kind that they will be brought to realise the main 
characters which distinguish stratified from unstratified 
