66 DISTRIBUTION OF THE MATERIALS OF THE EARTH. 



connected with the unstratified substances ; and, how- 

 ever interesting from their origin arid phenomena, 

 they occupy but very small spaces, and have no effect 

 in modifying the general surface. They are neces- 

 sarily posterior to the rocks which they intersect ; and 

 thus a judgment can sometimes be formed, within 

 certain limits, of the period at which they were pro- 

 duced. 



The last of the hard or rocky materials of the earth, 

 consists of those veins, also posterior to many of the 

 rocks, which are the repositories of metals, as well as 

 of many earthy minerals, and which, from their ceco- 

 nomical value, become objects of great importance. 



The loose materials of the earth are found depo- 

 sited over the surface ; generally concealing the rocks 

 from view, but distributed in a very unequal manner 

 in different places. Where they merely cover the 

 surfaces of the hills or plains, they only modify the 

 forms of those ; concealing asperities, filling valleys, 

 or obliterating lakes. But they often constitute large 

 portions of the surface, by themselves ; limiting this 

 remark, by geographical considerations, to the mean 

 level as determined by the sea. This is peculiarly the 

 case at the feet of high or extensive tracts of mountain 

 land ; and particularly where large rivers are found, 

 which, depositing the ruins they carry along, cause 

 the sea to retire, and form extensive plains. The 

 consideration of this very interesting branch of Geo- 

 logy, belongs to the history of alluvia, and to that of 

 the changes which the surface of the earth is daily 

 undergoing; and it will hereafter form a distinct 

 object of examination. It is unnecessary to extend 

 further a sketch which is merely intended as an intro- 

 duction to the succeeding parts of this work. 



