36 GEOGNOSY. ‘[Boox I. — 
North America, for example, a continuous shaded ridge is placed 

5 
1 
down the axis of the continent and marked “Rocky Mountains,’ — ; 
while the vast level or gently rolling prairies are left with no mark 
to distinguish them from the maritime plains of the eastern and 
southern states. In reality there is no such continuous mountain 
chain. The so-called “Rocky Mountains” consist of many inde- 
pendent, and sometimes widely separate ridges, having a general 
meridional trend rising above a vast plateau, which is itself 4000 or 
5000 feet in elevation. It is not these intermittent ridges which 
really form the great mass of the land in that region, but the 
widely extended lofty plateau, or rather succession of plateaux, which 
supports them. In Europe also the Alps form but a subordinate 
part of the total bulk of the land. If their materials could be 
spread out over the continent, it has been calculated that they would 
not increase its height more than about twenty-one feet. 
Attempts have been made to estimate the probable average 
height which would be attained if the various inequalities of the 
land could be levelled down. Humboldt estimated that the mean 
height of Europe must be about 671, of Asia 1132, of North America 
748, and of South America 1151 feet.‘ Herschel supposed the 
mean height of Africa to be 1800 feet. These figures, though 
based on the best data available at the time, are probably not very 
near the truth. In particular, the average height assigned to North 
America is evidently less than it should be; for the great plains 
west of the Mississippi valley reach an altitude of about 5000 feet, © 
and serve as the platform from which the mountain ranges rise. 
Recent calculations by G. Leipoldt give for the mean height of ; 
Europe 296:838 metres (973°628 feet)? It is very desirable that 
more reliable estimates should now be made for the whole globe, 4 
as furnishing a means of comparison between the relative bulk of — 
different continents, and the amount of material on which geological 
changes can be effected. . 
The highest elevation of the surface of the land is the summit 
of Mount Everest, in the Himalaya range (29,002 feet) ; the deepest 
depression not covered by water is that of the shores of the Dead 
Sea (1300 feet below sea-level). There are, however, many subaqueous 
portions of the land which sink to far greater depths. The bottom of 
the Caspian Sea, for instance, lies about 3000 feet below the general 
sea-level. 
There are two conspicuous junction-lines of the land with its 
overlying and surrounding envelopes. First, with the Air, expressed 
by the contours or relief of the land. Second, with the Sea, expressed 
by coast-lines. 
1 Asie Centrale, tom. 1, p. 168. _ * Physical Geography, p. 119. 
8 Die Mittlere Hohe Europas, Leipzig, 1874. In this work the mean height of 
Switzerland is put down as 1299'91 metres; Austria, 517°87 ; Italy, 517-17; Scandinavia, 
42810; France, 393°84; Great Britain, 21770; German Empire, 213'66; Russia, | 
167:09; Belgium, 163°36; Denmark (exclusive of Iceland), 35:20; the Netherlands 
(exclusive of Luxembourg and the tracts below sea-level), 9°61. 

