14 
AMERICAN GEOLOGY. 
sive cold and thin atmosphere. The causes which are now 
operative in excluding animals and vegetables from deep sea 
bottoms and high mountains, have also been operative in all 
periods of the earth’s history. 
§ 14. The earth’s surface did not receive its present configu¬ 
ration at its creation. Its mountains and valleys had no exist¬ 
ence in the original constitution of the globe. Even its highest 
mountains, the Himalayas, have been raised long since animals 
and vegetables were created; and oceans but lately rolled over 
lands which are now the highest points of continents. Power¬ 
ful agents have therefore been operative in these changes. 
The most effective of these agents are water and fire. Water 
is operative in many ways. It is a solvent of many of the 
materials composing the earth’s crust. The rocks are dissolved 
by water. It is the most effective when aided by heat arid 
pressure. In the deep parts of the earth’s crust, where there is 
both heat and pressure to aid it, silex is dissolved; and if it 
rises to the surface in a heated state, and there cools, its silex 
is deposited upon the soil or rocks, as at the Geysers of Iceland. 
This deposit is called siliceous sinter . Cold water readily dis¬ 
solves carbonic acid, which is diffused in the atmosphere and 
soil; and cold water, aided by carbonic acid, dissolves carbonate 
of lime and other carbonates, together with the oxides of iron, 
manganese, and phosphate of lime. Spring water, which holds 
them in solution below the surface and under pressure, deposits 
them at the surface. Incrustations of lime and porous beds of 
it, are formed around these springs. These deposits are 
called tufa or travertine. It should be noted that travertine is 
a rock formed on the dry land—it is a subaerial formation. 
Ochrey iron ore, mixed with carbonate of iron, and manganese 
are' also deposited around springs, and upon dry land, under 
similar conditions. All these are subaerial deposits, and should 
be distinguished from the subaqueous. Water percolating 
through the soil, holding in solution carbonic acid, dissolves 
carbonate of lime and iron, which it deposits on the coarse and 
fine materials, when they become cemented together. They 
