ROCKS. 257 



Observations made with extreme accuracy over large tracts 

 of land, show that erupted rocks have not been produced in 

 an irregular and unsystematic manner. In parts of the globe 

 most remote from one another, we often find that granite, 

 basalt, and diorite have exercised a regular and uniform meta- 

 morphic action, even in the minutest details, on the strata 

 of argillaceous slate, dense limestone, and the grains of quartz 

 in sandstones. As the same endogenous rock manifests almost 

 everywhere the same degree of activity, so, on the contrary, 

 different rocks belonging to the same class, whether to the 

 endogenous or the erupted, exhibit great differences in their 

 character. Intense heat has undoubtedly influenced all these 

 phenomena, but the degree of fluidity (the more or less 

 perfect mobility of the particles their more viscous com- 

 position,) has varied very considerably from the granite to 

 the basalt; whilst at different geological periods (or ineta- 

 morphic phases of the earth's crust.) other substances dissolved 

 in vapours, have issued from the interior of the earth, simul- 

 taneously with the eruption of granite, basalt, greenstone- 

 porphyry, and serpentine. This seems a fitting place again 

 to draw attention to the fact, that, according to the admirable 

 views of modern geognosy, the metamorphism of rocks is not 

 a mere phenomenon of contact, limited to the effect produced 

 by the apposition of two rocks, since it comprehends all the 

 generic phenomena that have accompanied the appearance of 

 a particular erupted mass. Even where there is no imme- 

 diate contact, the proximity of such a mass gives rise to 

 modifications of solidification, cohesion, granulation, and 

 crystallization. 



All eruptive rocks penetrate, as ramifying veins, either 

 into the sedimentary strata, or into other equally endogenous 

 masses ; but there is a special importance to be attached to the 

 difference manifested between phitonic rocks,* (granite, por- 

 phyry, and serpentine,) and those termed volcanic in the 

 strict sense of the word (as trachyte, basalt, and lava). The 

 rocks produced by the activity of our present volcanoes, 

 appear as band-like streams, but by the confluence of several 

 of them, they niav form an extended basin. Wherever it has 

 been possible to trace basaltic eruptions, they have generally 



* [Lycll, Principles of Geology, vol. iii. pp. 353 and 359.] Tr. 



S 



