244 



THE NATURE BOOK 



It has been found, however, that it would 

 not do to use mean sea-level indiscrimin- 

 atelv and at any place as a starting point, 

 and so the Ordnance Survey in preparing 

 the maps of our countr)' were compelled 

 to adopt a certain line, which they con- 

 sidered, after careful and prolonged obser- 

 \'ation, to represent the mean sea-level 

 at the Old Dock Sill at Liverpool. 



our feet. 





"THE STRATA . . . MAY BECOME TILTED INTO AN 

 INCLINED POSITION." 



It may be asked, by what signs do we 

 know that the Earth's surface is subject 

 to changes of level ? 



In many parts of our coasts, and more 

 particularly on the west coast of Scotland, 

 we meet with low platforms extending 

 seawards from inland cliffs. In all respects 

 they are like the beaches now being 

 formed on the shore. The sands and 

 gravels of which tliey are composed 

 contain the remains of marine shells ; yet 

 they are never covered by the highest 

 tides, but stand high and dry twenty-live 

 to fifty feet above the present sea-level. 

 In other countries ancient beaches have 

 been raised to a height of many hundreds 

 of feet.* 



While these may be looked upon as 

 proofs of the elevation of the land, forests 

 of oak, poplar, pine and other trees 

 which are now submerged under the sea 

 at high water, convince us that the land 

 has been depressed in other parts of our 

 coasts. 



On a grander scale we find ancient sea 

 deposits among the highest peaks of the 

 Alps and the Himalayas. We cannot 

 ♦ See "Waves and Their Work," p. 221. 



stop to inquire when and how these great 

 changes took place ; it is sufficient for 

 our purpose to know that the Earth con- 

 tains within itself forces sufficiently power- 

 ful to crumple the Earth's crust into 

 ridge and furrow, and continent and ocean 

 alike bear witness to the vast energies 

 existing;, but not slumbering, beneath 

 In the gradual exercise of these 

 energies mountains have their 

 origin. 



In tracing the history of a 

 mountain, let us assume that 

 the sea bottom is in process 

 of gradual elevation. 



The infant mountain has 

 scarcely emerged above the 

 waters before it is subject 

 to forces from the outside 

 which tend to reduce it to a 

 lower level. During growth 

 these same destructive agents 

 are at work, and they cease 

 not when it has attained its 

 full stature and strength. 



In the end they prevail, 

 and the mountain is once 

 again removed into the midst 

 of the sea to form " the dust of con- 

 tinents to be." 



Thus, like most of the operations of 

 Nature, the events run in cycles, and 

 return to the point from which they 

 started. 



In considering the cycle of events 

 through which mountains pass, we must 

 keep full in view these two conflicting 

 elements : one working from within 

 tending to uplift, the other from without 

 tending to destroy. 



The character and disposition ot the 

 material on which these forces pla}^ are 

 also important in determining the form 

 which the mountains assum.e. 



Some rocks, such as granites, have Ijcen 

 solidified, b}'^ cooling, from a state of 

 fusion. They are usually comj^act and 

 homogeneous, and offer few jioints fa\'our- 

 able to attack- Their weakness chiefly 

 lies in the cracks and joints, which develop 

 as the mass shrinks, and they are .sus- 

 ceptible to chemical decay. 



Other rocks such as sandstone, lime- 

 stone, shale and slate are commonly laid 

 down in layers or strata. It is seldom 

 they j)reserve the same characters through 



