J32 ORIGIN OF TABLELANDS. 



results that the tableland becomes surrounded by mountains. Some- 

 times one side rises faster than the other, and then as in the tableland 

 of Mexico, for instance we find the Pacific side rising rapidly in a 

 series of terraces, which indicate more or less stationary intervals, 

 while the tableland slopes somewhat towards the Atlantic coast. 



It is impossible to go into any part of the mountain scenery of 

 Wales, or the other higher districts of Britain, without realising that 

 the multitudinous peaks which occur at approximately the same level 

 are but the last relics of a plain which was originally continuous 

 between them ; sometimes, indeed, a succession of plains descending 

 one below another, may in this way be traced out by the eye. Yet so 

 ancient was their origin, and so powerful have been the denuding 

 agents acting upon them, that the mountains play the part of gigantic 

 earth-pillars, such as are seen in Colorado or the Tyrol, and are the 

 sole remaining points in tablelands which are now variously sculptured 

 into valleys and hills. 



Similarly the Secondary strata, which are less inclined than the 

 Primary rocks, and but little folded, rest upon each other so as to form 

 surfaces which are approximately plains, though the flocculent particles 

 of the unhardened clays have been swept away so as to give them, a lower 

 level than the more enduring limestones and sandstones. The British 

 Isles need but to be elevated some 600 or more feet to present the 

 essential characteristics of tablelands. It will thus be clear that a 

 tableland is a plain distinguished by its mode of origin, and is quite 

 independent of height ; for even where its level is relatively low, it is 

 in all respects the antithesis of what is termed a low plain. It may 

 be convenient here to put these characteristics in contrast, and say 

 that a tableland is the oldest part of a land, while a low plain is the 

 newest part of a land ; a tableland is formed in a region of predominant 

 anticlinal fold, while a low plain is formed in a synclinal fold; a 

 tableland consists of such rocks as form the fundamental structure of 

 the country, while low plains consist of detritus worn off from the 

 higher ground and deposited at a lower level ; tablelands are usually 

 dry, relatively barren regions carrying their rivers in deep narrow 

 valleys ; low plains are the fruitful populous portions of the earth, 

 carrying their rivers through valleys which are broad and shallow. No 

 region exemplifies the relations of these phenomena better than South 

 America. 



A Tableland in the Andes. High up in the Andes of Quito and 

 of Bolivia we find the peaks of the mountains planed away, and a 

 broad level surface presented ; which, in the cases of Desaguardero, is 

 upwards of 13,000 feet high and over 500 miles long. This tableland 

 was described by Mr. David Forbes, F.R.S. Near Arica the mountains 

 rise abruptly 3000 feet from the water's edge, and everywhere as we 

 ascend there is evidence of emergence from the ocean, and evaporation 

 of the sea, in the existence of deposits of salt. ist. At a height of 2500 

 feet to 3500 feet, beds of nitrate of soda run from ten to forty miles 

 inland. These beds were originally chloride of sodium decomposed 

 by carbonate of lime into chloride of lime and carbonate of soda, 



