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apparent in all the phenomena; but the degree of fluidity 

 has varied greatly in all of them from the granite to the 

 basalt: and at different geological epochs, eruptions of 

 granite, basalt, greenstone, porphyry, and serpentine, have 

 been accompanied by the issue of different substances in a 

 state of vapour. According to the views of modern geology, 

 the metamorphism of rocks is not confined to the effects of 

 simple contact, or of the juxta-position of two kinds of rock; 

 but it comprehends all the phsenomena that have accom- 

 panied the issuing forth of a particular erupted mass ; and 

 even where there has been no immediate contact, the mere 

 proximity of such a mass has frequently sufficed to produce 

 modifications in the cohesion of the particles and texture of 

 the rock, in the proportions of the silicious ingredients, and 

 in the forms of crystallization of the pre-existing rocks. 



All eruptive rocks penetrate as veins into sedimentary 

 strata, or into other previously existing endogenous masses ; 

 but there is an essential difference in this respect between 

 plutonic rocks, granites, porphyries, and serpentines, 

 and those called volcanic in the most restricted sense, tra- 

 chytes, basalts, and lavas. The rocks produced by the 

 still existing volcanic activity present themselves in narrow 

 streams, and do not form beds of any considerable breadth, 

 except where several meet together and unite in the same 

 basin. Where it has been possible to trace basaltic erup* 

 tions to great depths, they have always been found to termi- 

 nate in slender threads, of which examples may be seen in 

 three places in Germany, near Marksuhl, eight miles 

 from Eisenach, near Eschwege, on the banks of the Werra, 

 and at the Druidical stone on the Hollert road (Siegen), 

 In these cases, the basalt, injected through narrow orifices, 



