SURFACE OF THE GLOBE. 43 



Hence the lower elevations are rarely marked by 

 conspicuous asperities ; and, except under peculiar 

 circumstances of climate or exposure, or when formed 

 of certain indestructible rocks, they seldom display 

 the nature of their structure or contents. More com- 

 monly they are covered with that alluvial soil which 

 finds so firm a resting place on them ; and thus their 

 outlines are rounded and tame, while being at the 

 same time favourable to the uses of man, they are 

 covered with vegetation. If ever, in the lower hills, 

 the naked rock is displayed, it will be found at those 

 places where the elevated edges of the strata lie, and 

 where the geologist finds, if with difficulty yet most 

 securely, those indications of the internal structure of 

 a country which are of so much easier access in more 

 elevated regions. 



It is scarcely necessary to say, that between the 

 spiry and rugged mountain and the flat vacant plain, 

 every degree of altitude and every form of undulating 

 surface may exist. But even plains are sometimes 

 considerably elevated above the sea ; although it is 

 more usual to find them at low levels. In extent, 

 they vary from the enormous tracts of Asiatic Russia 

 and the sandy deserts of Africa, to the narrowest 

 limits of those which diversify the undulating lands 

 of England. In some instances, their forms depend 

 on the horizontal or even positions of the strata on 

 which they lie ; but, in many, they are the produce of 

 alluvial deposits, resulting from the joint action of the 

 sea and rivers. 



Every interval between two hills forms a valley ; 

 and thus the extent, the forms, the dispositions, and 

 the depths of vallies, are counterparts of the cor- 

 responding circumstances in the elevated lands by 

 which they are determined and bounded. In the most 



