380 EXTRUSION OF PLUTONIC ROCKS. 



upheavals, we are then more or less enveloped in the 

 domain of speculation. 



How did the great event come to pass? We ven- 

 ture to suggest that mountain ranges are simply ridges, 

 or creases, formed upon the earth's surface by com- 

 pression, and shrinkage occurring in its crust, as its 

 exterior surface cooled. Shrinkage caused a crack, or 

 fissure to form ; while compression caused plutonic 

 rocks, existing in a molten condition within the earth's 

 surface, to extrude, and these rising in a soft but not 

 in a fluid state, through the fissure, formed the heart, 

 or core, of the mountains and this is exactly what is 

 found to occur in the structural formation of all great 

 mountain ranges: the granites and other crystalline 

 rocks form the central and higher portions of the rid- 

 ges: that is to say the mountain peaks while the 

 sedimentary, or fossil bearing rocks, through which 

 they have been forced, form the exterior sides, and 

 bases the slopes of the hills in fact. 



We must here explain that the plutonic rocks (to 

 which allusion is made above) differ from the volcanic 

 rocks, in that the latter always seem to overflow in a 

 fluid state, like lava from volcanoes, and are generally 

 therefore found to be overlying the newer rocks through 

 which they have been ejected; whereas the plutonic 

 rocks (comprehending all the granites, certain porphyries, 

 and the crystalline schists) rarely or never do so. * 

 It seems probable therefore that they are forced up 

 from below by compression, in a soft or plastic state ; 

 they thus rise up aloft and form mountain peaks, 

 but do not overflow, like lava. Moreover, though 



* See passages in the Elements of Geology, by Sir Charles Lyeli, 

 pp. 7 and 8. 



