ROCKS 187 



inferred, not from anything which we have actually seen accom- 

 plished, but from the traces which it has left in the structure. 

 Under such circumstances, it need not surprise us to find that 

 the explanation is not always easy and obvious, but may be very 

 problematical, and that great differences of opinion may arise con- 

 cerning the rightful interpretation of a complex region. 



Here, as in all other provinces of geology, the historical stand- 

 point is the dominant one. Our object is to learn, not only the 

 agencies which have produced the structures and the way in which 

 they operated, but also the successive steps by which the structures 

 originated, the order in which they occurred, and their geological 

 date. Thus they may be coordinated into the great history of the 

 earth, which it is the main problem of geology to construct. 



ROCKS 



The distinction between a rock and a mineral is not always an 

 easy one for the beginner to grasp, yet it is essential that he 

 should do so. A Rock is any extensive constituent of the earth's 

 crust, which may consist, though rarely, of a single mineral, but in 

 the great majority of cases is a mechanical mixture of two or more 

 minerals. A rock thus has seldom a definite chemical compo- 

 sition, or crystalline form, or homogeneous internal structure. An 

 examination with the microscope almost always shows that a rock 

 is an aggregate of distinct minerals, which may be all of one kind, 

 or of many different kinds, in varying proportions. Rocks, then, 

 are mechanical mixtures, and their properties vary in proportion 

 to their various ingredients, while minerals are chemical com- 

 pounds (see p. 9). 



In ordinary speech the term rock is held to imply a certain 

 degree of solidity and hardness, but in geological usage the 

 word is not so restricted. Incoherent masses of sand and clay 

 are regarded as being rocks, quite as much as the hardest 

 granites. 



The classification of rocks is a very difficult and obscure prob- 

 lem, and would be so, even were our knowledge much more com- 



