OUTLINES OF GEOLOGY. 



27 



not been aqueous. In spite of this mode of examining the 

 relative date of formations, there are still many parts of the 

 crust of the earth which it is difficult to assign to precise 

 geological epochs. This may happen from various causes : 

 (1.) We may have no means of determining their exact 

 position. 



(2.) The same rock may occur in formations which, for 

 other reasons, we should consider as belonging to different 

 epochs. 



(3.) A rock may not belong to the series in which it is 

 found, but may have been subsequently introduced, cutting 

 through others more ancient than itself or being intercalated 

 among them. 



(4.) It is yet possible that the order in which formations 

 succeed each other, may not be identical in all parts of the 

 globe. No more than a part of Europe has been fully exa- 

 mined, but every new region which is explored seems to 

 confirm that the order which exists in that quarter of the 

 globe is, in its general features, the same which governs the 

 formations of other countries. 



The difficulties are enhanced by the fact, that whole 

 formations of great extent, which are found in some places, 

 are wanting in others. 



58. We obtain our knowledge of the order in which 

 rocks are superposed, by the outcrop of inclined strata or 

 beds ; by digging pits or sinking mines ; aud by the appear- 

 ances presented on the flanks of mountains, and the sides of 

 deep valleys. 



59. Some formations are going on under our own eyes, 

 and the date at which the causes that produce them began 

 to act, cannot be referred back more than a few centuries. 

 Such formations may be styled modern. There are others 

 which had assumed the character and position in which we 

 now find them, before the modern formations began. The 



