CHAP, xi The Bed of the Oceans 189 



ture of the globe, but one about 3 miles higher than the 

 other. The lower and larger is composed of broad gently 

 undulating plains rising into gentle ridges, and broken by 

 some abrupt peaks. It is divided into bay-like expanses by 

 the higher region, the slopes up to which are almost every- 

 where steep and often precipitous. The higher region is 

 smaller and more diversified, rising into numerous terraced 

 plateaux and rugged peaks. The whole of the low-lying 

 region and the lower slopes of the higher region are 

 entirely covered by the hydrosphere, only the plateaux and 

 peaks of the latter project above the water surface and 

 form the land. 



252. Sea-Level. The surface which naturally presents 

 itself for purposes of comparison in describing the con- 

 figuration of the Earth is that of the Ocean. This surface 

 is usually considered to be level, that is to say it is looked 

 on as having the exact form of the geoid ( 83) and being 

 concentric with it. The level of the sea at any place is 

 always varying on account of waves and tides. In con- 

 structing charts, all soundings of depth are corrected to 

 their value for a calm sea at the average low water of 

 spring-tides for the place** in question. Heights -on land 

 are measured from a datum-level, which differs in different 

 countries, but is usually the average height of the sea 

 at some selected place. The heights marked on an Ord- 

 nance Survey map of Great Britain are quite accurate with 

 regard to the datum-level (that of mean tide^ at Liverpool), 

 but are 8 inches too high compared with the average sea- 

 level round the island, and in certain plages are as much as 

 2 feet too high or too low compared with actual mean sea- 

 level. Many reasons exist for those small permanent dif- 

 ferences of level, such, for example, as heavy local rainfall, 

 or evaporation, the direction of prevailing winds or currents. 

 The greatest distortion of the sea-surface is, however, due 

 to the mobility of water and its readiness to yield to the 

 attraction of gravity. If the surface of the lithosphere were 

 smooth and its interior of uniform density, this property of 

 water would ensure a truly similar surface in the ocean. 

 The Elevated Regions projecting to unequal heights far 



