10 MODERN HIGH FARMING. 



Geologists agree in supposing that our globe was once a glowing 

 mass of fire, that the formation of the earth's crust is due to the 

 gradual process of cooling. They point to its figure and compare it 

 to that of a liquid rotatory body acted upon by gravity, and from 

 this deduce evidence of its original fluid state. The volcanoes actually 

 in a state of eruption prove the earth to have an internal temperature 

 independent of the heat of the sun ; and from observations made in 

 the deepest mines all over the world, it would appear that this tem- 

 perature, below a depth of one hundred feet, increases 1 Fahrenheit 

 in every twenty yards; so that, presuming the rate of increase to be 

 constant, at twenty thousand yards we should arrive at a low red 

 heat, and by descending still lower, this would be sufficiently in- 

 creased to maintain all kinds of minerals in a state of fusion. 



The different species of rocks of which the earth is composed, 

 may be divided into three groups. 



FIRST SANDSTONES.^These belong to every geological period, 

 are sedimentary and of infinite variety, though differing only in 

 the size of the grains of which they are composed and in their 

 degree of texture and compactness. When occurring in 

 connection with clays, marls, chalks, iron-ores, glauconite, or 

 felspar, in varying proportions, they undergo transformations 

 which convert them into vegetable soils. 



SECOND LIMESTONES. These are organically formed rocks, 

 made up of the remains of animal life, corals, shells and 

 bones, cemented and so intimately bound together as to neces- 

 sitate the minutest investigation for their recognition. 



THIRD GRANITE. These are igneous rocks, of volcanic origin, 

 and owe their formation to the cause of interior heat. They 

 are composed of numerous distinct orders of crystals such as 

 felspar, mica, gneiss, quartz and hornblende, mixed up 

 and not in regular beds. 



The history of these three great groups of rocks may therefore be 

 aptly termed the history of the earth, since their decomposition un- 

 der the combined influence of the atmosphere and water, during a 



