GEOLOGY. 491 



The land surface of the globe has been, for convenience of 

 discription, formed into two divisions, termed the old and new 

 world, in relation to their priority of habitation. Europe, Af- 

 rica, and Asia form the first ; and the great continent of Ame- 

 rica the second. All the great peninsulas in both, it has been 

 remarked, point towards the south ; and there are many minor 

 points of agreement in the disposition of the continents which 

 have been observed as common to both. All are variegated by 

 inequalities of the surface more or less conspicuous, as ranges 

 of lofty mountains, single isolated mountains or mountain groups, 

 hills, plains, and valleys. The direction of mountain groups is 

 generally according to the longitudinal dimension of the conti- 

 nents or islands in which they occur ; and the principal valleys 

 are at right angles on each side of this longitudinal line. 



Of the agents in nature which have produced these inequali- 

 ties, the atmosphere and water, both by their mechanical and 

 chemical action, seem to be the most efficient. Water falling 

 on the surface of mineral bodies of various composition, aided 

 by the action of the atmosphere, soon forms hollows in the more 

 easily decomposed parts ; these hollows form lakes ; lakes sur- 

 changed with water burst the barriers which confine them and 

 produce rivers ; these deepening their channels, form shelving 

 banks, which give an additional power to the waters which fall 

 from the atmosphere, and valleys are formed. According to the 

 nature of the mountain masses, the softer parts are washed or 

 crumble away, and thus are produced in the more untractable, 

 pointed pyramidal peaks, or in those more subject to the wasting 

 power, rounded eminences, or undulating surfaces. Thus the 

 present variegated surface of the earth is supposed, in the course 

 of ages, to have been produced by the mechanical and chemical 

 effects of air and water ; and, however gradual the operation, 

 in every successive season, according to Professor Playfair, some 

 change is produced for which no compensation is made, and some- 

 thing removed which is never to be replaced. Measurement has 

 ascertained that the present surface of the land, as compared to 

 the level of the sea, is gradually lowered; and the soil car- 

 ried down to the sea by rivers, and which mingles with their 

 waters, affords palpable evidence of this waste and disin- 

 tegration. The vast quantity of earthy matters thus trans- 

 ported to the basin of the ocean by the different rivers, carried 



