1 76 GROUPING OF VOLCANOES ON FISSURES. 



the streams flowed can be identified, another step is made in geological 

 demonstration of the nature of volcanic activity. 



Relation of Felsitic Bosses to Granitic Mountain Chains. It is 

 now well known that when granite is liquefied, and cooled so rapidly 

 as to put on a micro-crystalline texture, it becomes a grey or reddish 

 rock, which, to the eye, may be homogeneous or diversified with 

 crystals of felspar. In this state it is known as felsite, felstone, 

 petrosilex, or eurite ; and when it assumes a foliated schistose form is 

 known as halleflinta. Professor Judd has described on the flanks of 

 the Grampians 1 vast sheets of felsitic lavas of enormous thickness, 

 mixed with ashes, pumice, scoriae, volcanic bombs, and other evidences 

 of ancient volcanoes, the parent materials of which can only be sought 

 in the granite bosses of the Grampian chain. Similarly, in most of 

 the larger islands of the Inner Hebrides, granite peaks occur, which 

 are obviously the solidified cores of ancient and vast volcanoes, from 

 which flowed the surrounding lava streams of felsite or rhyolite, and 

 similar rock-substances. So that, without appealing to other instances, 

 we believe these to sufficiently establish the conclusion that granites 

 produced by metamorphism may be erupted, and pour out the rock- 

 material in all the forms which characterise volcanic eruptions. 



Linear Arrangement of Volcanoes. If we now examine a map of 

 the world, so as to observe the positions of existing volcanoes, they 

 will be found running for the most part in chains or lines, which are 

 regions of conspicuous upheaval, in some cases still undergoing per- 

 ceptible elevation. Thus the chain of the Andes and Central America 

 contains a multitude of well-known volcanoes, such as Corcobado, 

 Aconcagua, Villarica, Osorno in Chili, followed by Yiejo, Cotopaxi, 

 Coseguina, Popocatepetl, and many more, succeeded in the United 

 States by vast lava sheets in a region where volcanic activity is now 

 all but extinct. On the opposite side of the Pacific, lines of volcanoes 

 similarly extend through the Kurile islands southward by the Philip- 

 pines ; through Sumatra, Java, and adjacent regions ; and wherever 

 volcanoes exist they will be found to be in regions in which the force 

 of upheaval is manifest. Or, to take the case of a single volcano, we 

 have in Etna a vast mass where the base of the mountain consists of 

 lava streams and ashes, alternating with marine sediments ; proving 

 that the mountain, even if it did not originate in a submarine eruption, 

 has been greatly elevated during the progress of the eruptions, which 

 have resulted in its present bulk. And if there are no corresponding 

 evidences of changes of level of Vesuvius, the history of the Temple of 

 Jupiter Serapis 2 attests that certain changes have taken place in the 

 neighbourhood during the period of Vesuvian activity. And the 

 elevation of the coast of Chili, recorded by Mr. Darwin, was contem- 

 porary with volcanic activity in the adjacent mountains. So that we 

 have good ground for affirming that lateral pressure, similar to that 



1 Judd, "Secondary Rocks of Scotland," Quart. Jour. Geol. Soc., vol. xxx. 

 p. 220. 



* See Lyell, " Principles of Geology." 



