58 ORDER AND SUCCESSION OF THE BOOKS. 



external covering of a mass of molten matter. The oscilla- 

 tions of such a fluid tending towards different directions, will 

 sufficiently account for physical phenomena of the highest 

 relative interest and importance, such as the changes in the 

 external aspect of the earth by the elevation of continents 

 from the bed of the ocean, the upheaval of some portions of 

 the surface, the submergence of others, and the general 

 variety observable in its configuration. 



CEUST or THE EARTH. We are naturally directed to 

 this crust, or Erdrinde, (as the Germans term it,) as the 

 legitimate sphere of geological investigation. These terms 

 are used in a general sense, and without reference to any 

 theory as to its internal structure or contents. It will be 

 best understood if we describe it as that portion of our 

 planet which is accessible to human observation. It is com- 

 posed of a number of substances more fully to be described 

 hereafter, which under the name of rocks comprise every 

 element and combination, from loosely coherent beds of sand 

 and gravel, to crystalline strata and masses of granite ; and 

 from the ashes and scoriae of volcanic ejections, to the hardest 

 and most compact kinds of trap and basalt. 



The crust of the earth, although an infinitesimal quantity 

 when compared with the mass of the globe itself, is of 

 immense extent and importance. As regards its thickness, 

 which is estimated at about ten miles, it bears, in a physical 

 point of view, no greater relation to the mass of the globe 

 than that which would be offered by a film of gold leaf 

 coating the rind of an orange; regarded under another 

 aspect, it is the theatre of land and water, of mountain and 

 valley, of ocean, river, and lake, and affords a sphere for the 

 existence of the animal, vegetable, and mineral kingdoms. 



ORDER AND SUCCESSION or THE BOCKS. The constant 

 order of sequence of the several geological formations consti- 

 tutes one of the most important lessons which this science 

 teaches ; on this fact the first principles of geology are based. 

 At a period comparatively recent, the utmost ignorance pre- 

 vailed as to the structure of the earth. It was regarded as a 

 confused mass of inorganic matter, where heterogeneous 

 substances were mingled indiscriminately together. It was 

 known that at some places solid rocks rose to the surface, 

 and that in other regions they were absent ; but their order, 



