GEOLOGY. 187 



latter are the foot, the acclivity, and the summit. Lowland 

 comprises those extensive flat tracts which are almost entirely 

 destitute of small mountain groups. To the bottom of the 

 sea belong the flat, the rocky bottom, shoals, reefs, and 

 islands. It is only after a diligent study of the inequalities 

 just pointed out, that we can with advantage undertake to 

 explore the means employed by nature to produce them ; and 

 the first step is to proceed to the examination of the physical 

 causes of the slow, but unceasing changes of the globe. Obser- 

 vation teaches us, that most of the elevations and hollows we 

 meet with on the surface of the earth owe their origin to the 

 action of the atmosphere, to that of the ocean, and to volca- 

 nic fire. These powerful agents may be considered with 

 regard to their destroying, and, in consequence of this de- 

 struction, with regard to i\\e\x forming effects. 



All geologists are agreed that our present continents were 

 once covered with water. This is proved by the remains of 

 marine animals imbedded in the strata which lie on the sum- 

 mits of the highest mountains. The structure of the globe, 

 as far as we are acquainted with it from the intersections 

 made by rivers, by the action of the sea upon the coast, and 

 by mining operations, consists of beds of different kinds of 

 stone, which generally increase in thickness as we descend 

 deeper. Stratification, in its simplest form, may easily be 

 conceived, by placing a closed book with the back resting 

 upon the table, and raising the opposite edges a little ; the 

 book may represent a thick mineral bed, and the leaves a 

 series of strata. In nature we frequently find the strata 

 much broken, and thrown out of the original position. 

 Where any series of strata are wanting, a question naturally 

 arises, have they been carried away by some sudden inun- 

 dation, before the upper strata were deposited, or have 

 they never extended to that place ? In some instances it is 

 certain that the strata have been carried away from particu- 

 lar situations, as in some of the excavations which have 

 formed valleys, in which the strata that terminated on one 

 side of the valley may be discovered again in the hills on 

 the opposite side. The substances of which the strata are 

 composed, are argillaceous, calcareous, or siliceous earth, 

 which arc generally more or less intermixed or combined. 

 The strata of clay, or argillaceous strata, being water-tight, 

 give rise to springs, as they arrest the water that runs through. 



