CHAP, xii The Crust of the Earth 215 



proves that the surface of the sea must have sunk, or the 

 land must have risen since the waves eroded them. In the 

 south of Scandinavia and the south of England there are 

 many places where the sea now flows over what was dry 

 land even during historical time. This encroachment cannot 

 be due to erosion, as in some cases trunks of trees and walls 

 of buildings may be seen still standing under the shallow 

 water, and the necessary conclusion is that either the level 

 of the sea has risen or the land has sunk. It is difficult 

 to believe that for thousands of years the sea-level has been 

 slowly sinking around Scotland and Norway, and at the 

 same time slowly rising round England and Sweden, and the 

 only satisfactory explanation of the facts is that the land 

 must be undergoing gradual elevation in the north, and 

 gradual subsidence in the south of Britain and Scandinavia. 

 The regions of recent elevation and subsidence are marked 

 on Plate XV. Since the average height of the land is much 

 above sea-level, it is obvious that upheaval has been more 

 rapid on the whole than erosion, and more general in its 

 action over the Continental Area than subsidence. The 

 interpretation of the appearances of the Earth's crust, and 

 the utilisation of these to throw light on the past history of 

 the planet, is the subject-matter of geology. 



285. Rocks. The word rock is usually restricted to 

 the hard stony masses of cliffs and mountains, but the term 

 rock has a wider meaning. Geologists class as rocks all 

 substances which occur on or in the crust of the Earth and 

 have not been recently formed by the decay of living 

 creatures. Thus the term rock includes soil, sand, stones, 

 etc., but not bones nor dead leaves. . Some rocks are 

 uniform in structure like white marble or flint, but in most 

 cases they appear to be built up of small separate portions 

 which may be broken or rounded grains as in sandstone, 

 large crystals of different compounds as in granite ( 43), or 

 minute crystals so tightly packed as to be indistinguishable 

 by the unaided eye as in basalt. The grains of sandstone 

 or clay are merely fragments of older rocks that have been 

 broken and worn down before becoming cemented together 

 again ; but the regularly formed crystals are portions of pure 



