THE CRUST OF THE EARTH. 



nucleus upon which it is supported to the surface, constitutes the 

 subject of geology. 



15. Considering that however thin may be the crust of the earth 

 as compared with its diameter, its absolute thickness being at least 

 from 30 to 40 miles, and that this very far exceeds any depth 

 which is accessible to c|jrect observation, it might be imagined 

 that all attempts at an analysis of its structure would be vain 

 and futile. A circumstance nevertheless which at first view 

 would seem to throw difficulties insurmountable in the way of the 

 solution of this problem has, on the contrary, happily facilitated 

 it. This circumstance consists in certain local irregularities in 

 the actual state of the solid crust. What man could not do, the 

 accidental effects of internal forces has done for him. The solid 

 crust has been locally disrupted, so that its section in some cases 

 for many miles of depth has been turned upwards and exposed 

 to direct observation. It will be sufficient here to allude briefly 

 to this as one of the principal means, by which the structure of 

 the terrestrial crust has been ascertained. The point will be more 

 fully developed hereafter. 



16. To explain the condition and structure of the terrestrial 

 crust, we will suppose in the first instance a part of the earth's 

 surface to be selected, which throughout a considerable extent is 

 plane and level, and which has been subject to no derangement 

 of structure by internal convulsion, so that the materials which 

 form it, extending from the surface to the internal igneous fluid on 

 which it rests, have remained in their normal and original order 

 and state. If we imagine a vertical section of the crust in such a 

 situation to be made extending indefinitely downwards, it will be 

 found to consist of a series of strata superposed in a certain fixed 

 order, each stratum, taken in succession, consisting generally of 

 the same constituents in the same state. 



17. The matter composing these strata is called by geologists 

 BOCKS, a term used in this science in a sense somewhat different 

 from its common popular signification. ROCKS in the geological 

 sense does not necessarily imply masses of stone. It signifies any 

 agglomeration whatever of matter which may be found to enter 

 into the composition of the crust of the earth. In this sense clay 

 and sand come under the name of ROCKS as well as granite and 

 marble. 



18. Taking then the term rocks in this extended sense, the 

 successive strata composing the shell of the earth is found to 

 consist of different layers of rock, each layer being characterised 

 by rocks existing in a peculiar state of aggregation. 



The strata thus superposed, and extending from the funda- 

 mental layer which rests immediately upon the matter in igneous 



