682 DYNAMICAL GEOLOGY. 



L The Sun. — The Sun (1) causes, and must always have caused, 

 an increase of temperature over the globe from the poles to the 

 equator, as well as a variation of seasons. (2.) In the warmer 

 season it heats the soil to a varying depth, and then, through the 

 colder season, allows it to be chilled. At some depth, varying with 

 the latitude, the heat is uniform the year round : this depth in 

 temperate latitudes at the present time is 25 to 30 feet ; under the 

 equator, 3 to 4 feet ; in the frigid zone, but little more than under 

 the equator. (3.) The amount of heat derived from the sun, and 

 determining climate, varies with the extent and distribution of the 

 land (p. 45), and with its elevation, — a wide extent of high or 

 polar lands being an occasion of cold, and of low or tropical lands 

 an occasion of warm, climates. 



2. Chemical and mechanical action. — All chemical changes in which 

 there is condensation, as in liquids becoming solids, or gases liquids, 

 or either increasing their density, evolve heat. This is often an 

 effect of the natural decomposition of minerals or of vegetable or 

 animal matter. 



All mechanical action, as the beating of waves on a coast, the 

 falling of water in cascades or rain, the shaking of earthquakes, 

 sliding of rocks, motion of the atmosphere in winds, produces 

 heat whenever the action meets with resistance, on the principle 

 that motion corresponds to an amount of heat, and that heat 

 appears when the motion ceases. This source of heat has pro- 

 bably produced its effects, although it may be difficult to point 

 them out. 



3. Internal heat. — According to the results of geological research, 

 internal heat exhibits its effects over the whole surface, in volcanoes, 

 — earthquakes, — the metamorphism and consolidation of rocks, — 

 the elevation and subsidence of the earth's crust, raising and de- 

 pressing the continents or portions of them as well as islands, 

 and making mountain-chains. 



The proofs of the existence of a source of heat within the earth 

 are the following : — 



1. The spheroidal form of the earth is proof of original fluidity, 

 and therefore of a subsequent cooling over the exterior from a 

 state of igneous fluidity. 



2. The lowest rocks reached by geological exploration are crys- 

 talline rocks, — rocks which, if not once in igneous fusion, have at 

 least been crystallized through the aid of heat, and heat that must 

 have reached them from below. 



3. Borings for Artesian wells and shafts in mines have afforded a 

 means of taking the temperature of the earth at different depths ; 



