IGNEOUS ROCKS 129 



gneisses. These rocks are only found in regions of great earth 

 movement, such as mountain ranges, or in districts which are 

 the base of a great mountain range, the higher part of which has 

 been removed by denudation. 



It is only likely that the periods in which sediments were lifted, 

 folded, and broken, to renew the outstanding parts of the earth- 

 crust, should show signs of allied activity in the interior of the 

 earth. Such periods, and the places where there is evidence of 

 great movement, are connected with the outbreak of volcanoes 

 and the occurrence of earthquakes. The latter are tremblings 

 of the surface, often producing damage to buildings, great loss 

 of life, and disturbance of the atmosphere and the ocean. They 

 almost invariably accompany the eruption of volcanoes, but 

 even when the activity is not sufficient to cause volcanic 

 eruptions, earthquakes may occur. A study of the distribution 

 of their effects, the direction of transit of the waves they give 

 rise to, and the class of destruction they produce shows that in 

 some cases they are due to the fracture of rocks and the production 

 or intensification of faults. Disastrous as their effects on the 

 surface often are, the actual permanent movements of the surface 

 produced by them are often so slight that it is extremely difficult 

 to detect them at all. Thus the displacement produced along a 

 fault by any one earthquake may be extremely minute ; and the 

 production of the larger faults, in which the strata of one side may 

 have been lifted or lowered hundreds of feet, has probably been 

 an exceedingly slow process, occupying thousands of years. It 

 is also probable that this class of movement is proceeding quite 

 as rapidly at the present day as it ever has done. 



VOL. VI. 9 



