Evolution of the Earth. 83 



operated. Besides its influence in disintegrating rocks and 

 forming soil, glacial action has notably affected the conform- 

 ation of the earth's surface in many places, forming hills, 

 as we have said, excavating valleys, and even altering the 

 natural boundaries of the land and sea. Anglesea Island, 

 near Wales, for example, is supposed to have been separa- 

 ted from the main-land by the action of a great glacier which 

 descended from the mountains of the Lake Region, plough- 

 ing its way across the bed of what is now known as the 

 Irish Sea, and excavating a narrow channel in the soft rock 

 which previously connected Anglesea with the main-land.* 



The soil formed by these various agencies even as we see 

 it to-day — and at no earlier period was it more abundant — 

 is, however, scarcely more than a carpet covering here and 

 there the hard floor of rock. It is easy, almost anywhere, 

 to dig through this carpet ; or if you go down into the coal 

 or iron mines of Pennsylvania, or the marble quarries of 

 Vermont, you will see how thin is this superficial covering, 

 and that underneath it everywhere is the floor of rock. Pro- 

 ceeding with our investigations, we will now observe the 

 character of this underlying foundation, as it exists to- 

 day, and study the processes by which it has been formed. 



Present Condition of the Earth's Interior. Let us 

 first say a word, however, as to the present condition of the 

 earth's interior. Assuming that the world was originally a 

 mass of fiery matter, it Avas formerly believed that as the 

 process of cooling would naturally go on most rapidly on the 

 surface, the interior must still be in a molten or fluid con- 

 dition. As we descend in mines or quarries beneath the 

 earth's surface, we find that the temperature increases about 

 100 degrees for every mile of perpendicular descent. | As 

 we ascend a mountain, on the other hand, the temperature 

 falls. Upon the Alps, in summer, this fall amounts to about 

 one degree in 439 feet of perpendicular ascent, and in win- 

 ter, one degree in every 290 feet. Even on the equator, 

 therefore, if we ascend high enough, we may reach a region 

 of perpetual winter. It is only within a narrow range, ver- 

 tically, over the surface of the earth that animal or vegeta- 

 ble life is possible. The increasing heat as we descend into 



* Ramsey— " Earth-Sculpture." 



t The observed changes vary in different mines from about one degree in 50 

 feet to one degree in 75 feet. The deepest descent into the earth at which ex- 

 periment has been made is about 3000 feet. 



