134 ORIGIN OF LOW PLAINS. 



level becomes lower, and the two parallel mountain-chains which 

 border them diverge from each other so as to make the tableland 

 broader. 



It is almost impossible now to discover what share tablelands may 

 have had in the ancient geological history of the British region ; 

 but it is not difficult to see from the present position of the out- 

 crops, and from the manner in which deposits succeed each other, 

 that the history of British scenery is mainly the history of tableland 

 phenomena. 



Low Plains. There are, however, some formations which especi- 

 ally suggest low plains, and it may conduce to clearness if we briefly 

 notice the typical characters of the low plains of South America. 

 For since these sediments are so obviously the wreck and waste of the 

 Andes, slowly worn into mud and sand as the mountains rose, they are 

 typical of all plains which lie on the flanks of mountains, or in 

 synclinal folds. The whole surface of the pampas still retains salt 

 enough to render the streams saline in time of drought. Sometimes, 

 as at Bahia Blanca, salt covers the country like hoar frost for a thick- 

 ness of a quarter of an inch, and sometimes the sandstones are bound 

 together with salt. The material of the pampas is a red earth or mud, 

 which sometimes becomes compact marly rock. Pebbles are only 

 found at the foot of the mountains, and between the mountains and 

 the pampas mud the subsoil is sandy. Near Buenos Ayres the pampas 

 mud is upwards of 200 feet thick. Such plains may be matched in 

 Europe by the Russian steppes, the plains of Hungary, Lombardy, and 

 in our own country by little areas like the Carse of Stirling. Though 

 the fenland of Cambridgeshire is a plain, the deposits upon it are so 

 thin that it can scarcely claim to be an example of a low plain. When, 

 however, we look back in time, many formations suggest an origin 

 like that of low plains, and among such may be mentioned the Coal 

 Measures, and the various secondary and tertiary clays which yield 

 the remains of land vegetation, or of terrestrial reptiles. It may per- 

 haps be doubtful whether the land surface indicated by the Trias was 

 low or high; but, widely spread over Europe, it must have been a 

 plain, during its period of elevation, as conspicuous as any which 

 marks the existing surface of the earth. 



Lakes. Nearly all the great lakes of the world owe their existence 

 to direct upheaval of the ocean floor. If the English Channel were 

 to be raised two or three hundred feet, a number of small lakes would 

 extend along the deeper part of its bed. If Southern Europe were 

 to be somewhat upheaved, then the Black Sea would be perfectly 

 isolated from the Mediterranean. Similarly the Mediterranean might 

 be separated from the Atlantic, and at the same time the level of the 

 Mediterranean would be so far lowered by the draining off of the 

 waters by the Straits of Gibraltar, that the shallow sea between Tunis 

 and Italy would become dry, and the Adriatic would be surrounded 

 by land. Thus, merely by elevation to the amount of a thousand 

 feet, the Mediterranean might be converted into a chain of lakes. On 

 the other hand, if the south of Europe were to be somewhat depressed, 



