PHYSICAL GEOGRAPHY. 



produced within the interior of the earth. As 

 these become enlarged, their roofs, from failure 

 of support, will sometimes give way with a sudden 

 collapse. Such is not impossibly the origin of 

 many earthquake shocks. When we know that 

 even on the surface the explosion of a powder- 

 magazine sometimes gives rise to a tremor of the 

 ground, which is felt at a distance of several miles, 

 we may conceive how the collapse of one of these 

 underground cavities, and the consequent rushing 

 together of thousands of tons of rock, may send 

 a pulsation for many miles through the elastic 

 crust of the earth. 



The slow rising of some parts of the surface 

 is accounted for by supposing that among the 

 internal movements of the earth a great mass of 

 rock may gradually have its temperature raised, 

 and thus be expanded so as to push up the crust 

 lying above it. In like manner, if the rock cools 

 down, a slow depression of the overlying region 

 will be the result. But without supposing an 

 actual increase of temperature in any part, both 

 upheaval and subsidence may be accounted for by 

 the gradual cooling of the earth as a whole. As 

 the nucleus shrinks in dimensions, the outer coat 

 becomes too large for it, as it were, and has to 

 accommodate itself by going into frumples, bending 

 inwards in one part, and outwards in another. 

 The inequalities on the surface of the globe have 

 thus a similar origin to those on a shrivelled 

 apple ; or, to vary the figure, continents and sea- 

 beds, mountains and valleys, are the wrinkles that 

 mark the aging of mother earth. 



PLAINS, VALLEYS, AND OTHER DEPRESSIONS. 



The plains, or level portions of the earth's sur- 

 face, form a feature in its physical aspect equally 

 important with that presented by its mountain- 

 systems. The name, is given to extensive tracts 

 whose surface in the main is level, or but slightly 

 broken by elevations and depressions. They are 

 found at all elevations above the sea, and of every 

 degree of fertility ; from the exuberant tropical 

 delta just emerging from the water, to the irre- 

 claimable sterility of the desert of ever-shifting 

 sand. 



The noblest of these expanses are the river- 

 plains of the New World, drained by such waters 

 as the Mississippi, the Amazon, and La Plata. 

 Much of the Mississippi plain is rolling or undu- 

 lating in its surface, well watered by minor rivers, 

 exhibiting broad grassy prairies and extensive 

 pine-forests. In South America we have first the 

 low belt of country skirting the shores of the Pacific, 

 from 50 to loo miles in width, and about 4000 in 

 length, fertile at its extremities, but in the middle 

 sandy and arid ; next, the basin of the Orinoco, 

 consisting of extensive plains called llanos, either 

 destitute of wood, or merely dotted with trees, but 

 covered during part of the year with tall herbage ; 

 then the basin of the Amazon, a vast plain, 

 embracing a surface of nearly 2,000,000 square 

 miles, possessing a rich soil and humid climate, 

 and almost entirely covered with dense forests 

 and impenetrable jungle-marshes by the river- 

 sides ; and, lastly, the great Valley of the Plata, 

 occupied chiefly by open plains called pampas, in 

 some parts saline and barren, but in general 

 clothed with weeds, thistles, and tall grasses. 

 Next, in order of importance, is that section of 



Europe extending from the German Sea, through 

 North Germany and Russia, towards the Ura' 

 Mountains, presenting indifferently tracts of heath, 

 sand, and open pasture, and regarded by geo^ 

 graphers as one vast plain. So flat is the general 

 profile of this region, that it has been remarked, 

 'it is possible to draw a line from London to 

 Moscow, which would not perceptibly vary from a 

 dead level!' Passing the Ural ridge, a plain of 

 still greater dimensions stretches onward through 

 Siberia, towards the shores of the Pacific. This 

 region is of no great elevation, and, though diver- 

 sified by occasional heights, consists chiefly of 

 gravelly steppes, covered with coarse herbage, 

 lakes, and morasses. In Africa, the northern and 

 central portion, so far as explored, appears to be 

 a vast expanse of Sahara, or sandy desert, broken 

 at scanty intervals by oases of life and verdure. 



Certain minor tracts in these wide expanses 

 receive distinctive designations. These are the ver- 

 dant prairies of North America, already noticed, 

 the pampas and llanos of South America, the steppes 

 of Asia and Northern Europe, the tundras or bog- 

 marshes of Siberia, the grassy karoos of Southern 

 Africa, the tangled jungles of India, the alluvial 

 straths or dales of our island, and the low muddy, 

 but gradually increasing deltas of such rivers as 

 the Ganges, Nile, Niger, and Mississippi. To 

 lesser flats and depressions as valleys, glens, 

 ravines, &c. which give character to the land- 

 scape of particular districts, our space will not 

 permit us to refer. 



THE OCEAN. 



The ocean, though in fact a single mass of fluid 

 resting in the hollows of the solid crust, sur- 

 rounding the dry land on all sides, and indenting 

 it with numerous bays and gulfs, is generally 

 divided by geographers into the following great 

 basins : The Pacific Ocean, 1 1,000 miles in length 

 from east to west, and 8000 in breadth, covering 

 an area of 50,000,000 square miles ; the Atlantic^ 

 8600 miles in length from north to south, and 

 from 1890 to 5400 in breadth, covering about 

 25,000,000 square miles ; the Indian Ocean, lying 

 between 40 south, and 25 north latitude, is about 

 4500 miles in length, and as many in breadth, 

 covering a surface of 17,000,000 square miles ; the 

 Antarctic Ocean, lying round the south pole, and 

 joining the Indian Ocean in the latitude of 40 

 south, and the Pacific in 50, embraces an area 

 inclusive of whatever land it may contain of 

 30,000,000 square miles ; and the Arctic Ocean, 

 which surrounds the north pole, and lies to the 

 north of Asia and America, having a circuit of 

 about 8400 miles. Besides these great basins, 

 there are other seas of considerable extent, as the 

 Mediterranean, covering an area of 1,000,000 

 square miles; the German Ocean, 153,700; the 

 Baltic, 134,900 ; the Black Sea, with its subordi- 

 nate gulfs and branches, 181,000; but these and 

 other minor sections will be more appropriately 

 described when we come to treat of the respective 

 countries (Volume II.) with which they are politic- 

 ally as well as physically associated. 



Respecting the depth of the ocean, our know- 

 ledge has recently become much more definite 

 and certain, owing to improvements in deep-sea 

 sounding. The floor of the North Atlantic, in 

 particular, is nearly as well known as the most of 



