148 MINERALS AND GEOLOGY 



cavities here and there, filled with molten or fluid matter. The 

 more commonly received opinion, again, infers the surface rooks 

 technically known as the earth's crust to extend downward to a 

 more limited depth, whilst the whole of the internal portion is in a 

 condition of igneous fluidity. These views are practically identical, 

 in so far as they assume the presence of molten or incandescent matter, 

 and the existence of a high temperature, at a certain depth beneath 

 the surface rocks ; and they are sustained, more especially, by the 

 following data : 



(1.) Careful observations made in various parts of the world, shew that a 

 constant temperature is maintained throughout all periods of the year, at a 

 certain depth beneath the earth's surface. The depth varies in different 

 localities, and especially where different kinds of rock occur, but it averages 

 in temperate climates about 100 feet. At lower levels the temperature is 

 found invariably to increase with increase of depth. The ratio of increase is 

 not uniform, being greater or more rapid in some places than in others : but 

 an actual and marked rise of the thermometer from point to point, below the 

 zone of constant temperature, is always observable. The mean ratio of in- 

 crease, at the limited depths to which researches have been carried, may be 

 assumed to equal 1 Fahrenheit for each descent of 60 feet. At this ratio 

 even, and we may reasonably infer that it would be much accelerated at lower 

 levels, a temperature sufficiently high to maintain most mineral substances in 

 a state of fusion, or in part even in a vaporous condition, would soon be 

 reached. 



(2. ) Water brought to the surface from great depths by narrow bore-holes 

 commonly known as Artesian wells, always exhibit a higher temperature than 

 the mean temperature of the locality ; and if the boring be increased in depth, 

 the temperature of the water becomes also increased. 



(3. ) Active volcanoes, which may be regarded as channels of communication 

 between the surface and the internal parts of the earth, are more or less con- 

 stantly pouring forth, from unknown depths, vast streams of molten rock or 

 lava, accompanied by other products of igneous action. About two hundred 

 and seventy volcanoes are now known to be from time to time in eruption, and 

 many others are apparently in a permanently quiescent state. Eruptions also 

 frequently take place on the bed of the sea. 



(4.) Certain rock-masses, in districts now remote from centres of volcanic 

 action, have evidently been forced 'upwards, from deeply-seated sources, in a 

 molten or more or less incandescent state, amongst previously consolidated 

 rocks. The latter exhibit at the points of contact, and for some distance be- 

 yond, changes of colour, and other effects, that can only have resulted from 

 the direct or indirect action of heat. These effects are not seen in all cases of 

 rock-intrusion, but in the great majority of instances they have undoubtedly 

 occurred. 



