TEMPERATURE. 239 



least in a soft state, these circumstances also lead us to conclude 

 that at one time the earth was in an igneous and fluid state. This 

 hypothesis has been more readily admitted, as the temperature 

 of the earth increases below the surface. At a depth of from 

 60-80 feet the sun's heat has no influence, the same tempera- 

 ture prevails throughout the year, and it corresponds to the 

 mean annual temperature of the place. But if we go down still 

 deeper into the earth, the temperature increases about 1 Cent, 

 (or l-8 Fahr.) for every 100 feet. The observations of this 

 increase have hitherto been made only to a depth of about 2000 

 feet, so that we have only suppositions as to the conditions of 

 temperature at greater depths ; but springs of water are known 

 whose temperature reaches the boiling-point, and volcanoes cast 

 forth masses of burning lava; so that the subterranean locality 

 from which they proceed must have an extremely high tempera- 

 ture. If the heat increases in the same proportion below 2000 

 feet, we should find at 9000 feet the temperature of boiling 

 water; at 100,000 feet a heat of 1000 Cent, (or 1800 Fahr.), 

 which would melt many rocks ; and at 200,000 feet every thing 

 would be in fusion. Hence the Plutonists, or adherents of the 

 igneous theory, consider the interior of the earth to be a mass 

 of fluid lava, and regard hot springs and volcanic phenomena as 

 connected with that state of internal heat. Thus it was easy to 

 suppose that the whole earth was originally in a state of igneous 

 fusion, and that a firm crust had gradually been formed by its 

 cooling down in space, during which process the minerals be- 

 came solidified in a definite sequence, according to the degree of 

 heat required for the fusion of each kind. 



When the temperature had become so far lowered that water 

 could rest on the globe, thus producing the primaeval ocean, 

 stratified rocks began to be formed, a mass of materials being 

 accumulated at the bottom, and deposited in beds and strata. 

 From time to time fissures occurred in the crust of the earth, 

 from which issued the fused siliceous rocks, breaking through, 

 compressing, and even overturning the stratified masses, and so 

 forming the immense mountains which traverse the centre of 

 Switzerland. By the action of these protruded rock-masses the 

 surrounding strata were materially altered, and became meta- 

 morphic. Here and there those rocks which had been originally 



