MEANING OF THE TERM "ROCK." 19 



can only form hypotheses. And it is this crust which con- 

 stitutes the great theme of geological investigation. What 

 is the nature of the rocks of which it is composed 1 how are 

 they arranged 1 hy what agencies have they been formed 1 

 what changes are they now undergoing ? and, reasoning from 

 the known to the unknown, what changes do they seem to 

 have undergone in former periods ? If we can answer all 

 these questions, or approach to anything like a reasonable 

 answer, we then present something like a history of our 

 planet; and such a history is the aim and object of all 

 sound geology. 



\Ve have said that this earth-crust consists of rocks and 

 rock-formations ; and here we must explain that the term 

 " rock" is applied by geologists to all the solid substances 

 that enter into its composition. And there is good reason 

 for this usage. The sand and gravel of the sea-shore are 

 but comminuted rock-matters derived from the cliffs above; 

 the sands and loams and clays of the valleys are merely 

 rock-debris, worn and washed in course of ages from the 

 hills and uplands. Be it gravel or sand, clay or mud, all 

 are alike known to the geologist as " rocks;" and there can 

 be no doubt that, were these loose and soft matters con- 

 solidated by pressure or other agency, they would become 

 again compact and hard, like the rock-masses from which 

 they were originally derived. It is necessary, then, to bear 

 in mind this technical use of the term " rock ; " and the 

 least reflection upon the changes (mechanical and chemical) 

 which all rock-matter is incessantly undergoing will show 

 the appropriateness of the application. 



Understanding, then, what geologists mean by the term 

 "rock," and bearing in mind that their labours are re- 

 stricted to the accessible crust, let us inquire a little more 

 narrowly into the nature of the rocks of which this crust is 

 composed, and the modes of their arrangement. Wher- 



