62 VULCANISM ITS NATURE AND FUNCTION. 



abundantly manifested in various regions of the globe 

 here in isolated centres like those of the Mediterranean 

 and Iceland, there in linear directions like the Andes, and 

 occasionally over wide areas like the Indian and Chinese 

 Archipelagoes. At the present moment there are between 

 three and four hundred active volcanoes (with as many 

 more in a dormant or semi-extinct state), chiefly fringing, 

 as it were, the Pacific or scattered over its surface. In the 

 latter instance they appear in insular centres, as in the 

 Sandwich and other groups ; in the former they occur in 

 great lines, as in the Andean, Mexican, and Columbian 

 Mountains, the Aleutian Islands, Kamtschatka, Japan, the 

 Philippine Islands, and the Indian Archipelago. Com- 

 pared with these the display of volcanic energy in other 

 regions is insignificant ; while over immense tracts like 

 the north of Asia and Europe, and the Atlantic slopes of 

 both Americas, the internal forces have been still and sta- 

 tionary for ages. We know from the mountain-ranges of 

 these regions that the forces once were there ; we see their 

 effects in the primary granitic mountains, in the secondary 

 hills of basalt and greenstone, and in the tertiary domes of 

 trachytic lava;* but of the law that has regulated this shift- 

 ing from area to area, and now restricted it to its present 



* It may be of use to the general reader to mention that the principal 

 rocks in recent volcanic hills are lavas of various aspect and compact- 

 ness ; tufas, or consolidated cindery matters ; pumice, or light vesicular 

 lava; obsidians, of glassy texture; and trachytes, or granular-crystalline 

 masses : and that their differences depend partly on chemical composi- 

 tion, but chiefly on the rapidity with which they have been cooled 

 rapid cooling producing a compact glassy texture, and slow cooling the 

 reverse. The rocks of the secondary hills, on the other hand, though 

 originally consisting of the same volcanic ejections, are now converted 

 into crystalline greenstones, basalts, felstones, the softer trap-tuffs, and 

 amygdaloid*, or those whose original vesicles have got filled with almond- 

 shaped infiltrations of lime-spar, agate, and other minerals. In the older 

 mountains the conversions are still more complete, and highly crystalline 

 granites, syenites, and porphyries are the prevailing compounds. 



