Geology 



their characters to sedimentation they are known as 

 sedimentary rocks, and since they occur in more or less 

 definite layers they are also sometimes called stratified 

 rocks. They form the second of the three great classes. 



In mountainous regions and in other places where 

 the rocks of the Earth's crust have been greatly 

 disturbed, and where intense pressure and a high 

 temperature have in consequence been developed, rocks 

 of both igneous and sedimentary origin have under- 

 gone changes which have impressed upon them new 

 characters. So profound is the change in some in- 

 stances that it is impossible to ascertain what the 

 original rock was like before alteration. To these rocks 

 which have had new characters impressed upon them by 

 pressure and heat, the term metamorphic is applied. 



Thus we have three great classes of rocks (i) 

 Igneous, (2) Sedimentary, (3) Metamorphic. 



We will now examine some of these rocks with the 

 object of finding out what kind of material they contain, 

 and in what manner their particles are held together, so 

 as to give us a general idea of their structure, leaving 

 the question of the way in which they have been formed 

 until we have had an opportunity of examining them in 

 the field, for it is there, if anywhere, that we shall 

 find the clues which will enable us to trace them to their 

 sources and to read the different chapters of their history. 



On examining a piece of granite we at once notice 

 that it is built up of several different kinds of matter. 

 There are some parts which are pink, others which are 

 clear and glassy in appearance, while the whole surface 

 is speckled with dark brown, or almost black, particles 

 which glitter strongly in the light. 



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