CHEMISTRY OF THE EARTH. 207 



attest the connection between great accumulation and plutonic phe 

 noinena. 



§ 55. Coming uo\y to modern volcanoes, we find them in their greatest 

 activity in oceanic regions, where subsidence and accumulation are still 

 going on. Of the two continental regions akeady pointed out, that 

 along the Mediterranean basin is marked by an accumulation of meso- 

 zoic and tertiary sediments, 20,000 feet or more in thickness. It is evi- 

 dent that the great mountain zone, which includes the Pyrenees, the 

 Alps, the Caucasus and the Himalayah, was, during the later second- 

 ary and tertiary periods, a basin in which vast accumulations of sedi- 

 ments were taking place, as in the Appalachian belt during the paleozoic 

 times. Turning now to the other continental region, the American 

 Pacific slope, similar evidences of great accumulations during the same 

 periods are found throughout its whole extent, showing that the great 

 Pacific mountain belt of iSTorth and South America, with its attendant 

 volcanoes, is, in the main, the geological equivalent or counterpart of 

 the great east and west belt of the eastern world. 



It is to be remarked that the volcanic vents are seldom immediately 

 along the lines of greatest accumulation, but appear around and at cer- 

 tain distances therefrom. The question of the duration of volcanic 

 activity in a given region is one of great interest, which cannot, for 

 want of time, be considered here. It appears probable that the great 

 manifestations of volcanic force belong to the period of depression of 

 the area of sedimentation, if we may judge from the energy and copi- 

 ousness of the eruptions of island volcanoes, although the activity is 

 still prolonged after the period of elevation. 



As regards the geological importance of volcanic and earthquake 

 phenomena, their significance is but local and accidental. Volcanoes 

 and earthquakes are and always have been confined to limited areas of 

 the earth's surface, and the products of volcanic action luake up but a 

 small portion of the solid crust of the globe. Great mountains and 

 mountain chains are not volcanic either in their nature or their origin, 

 though sometimes crowned by volcanic cones; nor are earthquakes and 

 volcanoes to be looked ui)on as anything more than incidental attend- 

 ants upon the great agencies which are slowly but constantly raising 

 and depressing continents. 



