216 GEOLOGY DIAGRAM 20. 



others they are rare. When any are discovered which do not 

 resemble those which are commonly observed, or when they are 

 found in formations where they are very rare, they ought always 

 to be preserved, when possible, to be shown to persons who 

 know and can appreciate their scientific value. 



Mountains. The central mass of mountains is generally com- 

 posed of granite, while sedimentary strata are found on their 

 flanks and in their valleys. The snow and rain which accumu- 

 late on the highest mountains sometimes form great masses of 

 ice, which may be several miles long and broad, and are called 

 glaciers. The thickness of these masses of ice is sometimes con- 

 siderable, and they contain great cracks called crevasses into 

 which there is danger of falling when they are hidden by newly 

 fallen snow. 



While the fall of snow and rain increase the glacier, it par- 

 tially melts in the sun, and rivers which run into the valle} r s 

 always flow from the foot of a glacier. In spring when the 

 rain which has fallen on the mountains melts away, these rivers 

 are transformed into torrents. Their force then is irresistable, 

 and they carry away with them earth, pebbles, and rocks from 

 the highest parts of the valleys, and carry all this mass into 

 lower lying districts, where they form in time, immense fertile 

 plains. Consequently elevated countries are always being more 

 and more worn away by the action of the rain, and the melting 

 of the snow ; and this action is called erosion. The waters of 

 heaven thus remove, little by little, and by piecemeal, the soil 

 from the sides of mountains, just as the waters of the sea 

 gradually eat away the land on many coasts by another kind of 

 erosion. 



Springs. The water which results from the melting of snow 

 and ice, is not the only water which flows on the surface of the 

 earth. The greater part is supplied from springs which appear 

 to rise from beneath beds of earth. The fact is that all the rain 

 which falls does not flow into the brooks, nor dry up in the air. 

 When much rain falls, it always penetrates and infiltrates into 

 the ground, till it reaches a layer which it is unable to traverse. 



