INTRODUCTION. 21 



in all countries, and which must change both the plants 

 and the animals, is the destruction of lakes and pools. 

 In mountainous countries, that arises from the rivers 

 which are discharged by the lakes. The beaches 

 which had once been the margins of the water, can 

 often be traced along the sides of valleys that are now 

 dry, or which, at most, contain but a small rivulet ; 

 and in other cases the river, after having mined its way 

 through the softer strata, is arrested by hard rock 

 near the lake. Scotland, Wales, Switzerland, the slopes 

 of the Andes, all mountainous countries in fact, abound 

 with instances of this description ; and those countries 

 which at one time were nearly covered with water, are 

 so completely drained by those natural changes, as at 

 another to contain hardly a drop, and thus become 

 desarts : in which state both their plants and their ani- 

 mals must undergo a change. 



Sometimes, again, the land becomes parched through 

 the want of rain, to such a degree that the plants are 

 all withered, and the rain, when it does come, does 

 not penetrate into the soil. When that is the case 

 the quality of the vegetation changes, and in extreme 

 cases, wholly disappears. In the progress of this 

 change, as plants become fewer in number, they be- 

 come strongly impregnated with salt. The oil and 

 water which they contain are dried up by the heat, 

 and the charcoal, alkalis, and acids unite into new com- 

 binations, which are unfavourable to ordinary vegeta- 

 tion. The acridity augments, and at last nothing is left 

 but a barren sand covered with a crust of salt. 



We have one of the most remarkable instances of 

 this kind of change on the northern parts of all the con- 



