[3] 



down by the subsequent action upon them of air and water. This 

 first rock formation is termed by geologists the Plutonic (from Pluto, 

 monarch of hell), on account of its being the result of intense heat, and 

 not, as is the case with all other rock formations, laid down in layers by 

 water. Whether the Plutonic rock forms a solid centre to our earth is 

 matter of uncertainty ; but all are agreed that the internal heat of our 

 planet, whether caused by the friction of the particles of a solid sub- 

 stance or by a molten fluid, is still, even in these later times, intense, 

 In boring through the earth's crust, the average increase in temperature 

 for every fifty feet of descent, after the first hundred feet from the 

 surface, is one degree Fahr., which would give us, at a depth of 125 

 miles, sufficient heat to melt most of the rocks. This intense internal 

 heat has generated, in times long gone by, enormous forces, by which 

 rocks of all ages have been raised and depressed, twisted and distorted, 

 broken and forced out of position, and forcibly compressed, so as to 

 eventually cause most important changes of surface level. 



The next class of rock-formation is totally different from the 

 Plutonic, or unstratified series, in that it is the result of the wear and 

 tear of the surface when acted upon by air and water, and is laid down, 

 in the first instance, by water, as sediment. Water, in the forms of 

 seas, rivers, rain, and ice, has been the chief agent in the arrangement 

 of all the stratified rocks, the determination of the earth's contour, the 

 direction of valleys, and, in fact, the regulation of the whole physical 

 geography of the visible portion of the earth. With the help of this 

 mighty agent, so soon as the earth had become sufficiently cool to 

 permit condensation to take place in its vapoury envelope, the ceaseless 

 wear and tear of the Plutonic and, subsequently, of all other rocks, 

 which has accumulated so vast a mass of material, commenced. Large 

 volumes of water were gradually deposited, without intermission, until 

 permanent seas and rivers had become established, and the new process 

 of stratification, which was henceforth destined to shape the crust of 

 the earth and to provide the conditions of life, commenced to operate. 

 This action is taking place daily in rivers and seas, as we may observe 

 at any time. On the tops of mountains the same action is in operation, 

 though under different conditions, snow and ice splitting fragments from 

 the rocks to be borne away as grit into the valleys by impetuous torrents 

 and deposited in other places. Within the Polar circles ice on a 

 grander scale is levelling down the land ; glaciers, covering thousands 

 of square miles, are slowly sliding down the valleys, grinding their 



