40 RICIITIIOFEN NATURAL SYSTEM 



on local occurrences, and their general validity not being proved, the theories must 

 necessarily have a great deal of uncertainty, which will only gradually be dispelled by 

 the advancing knowledge of the geology of the globe. 



The correlation of age and texture, as resulting from observations made in 

 Europe, will occupy us first. Granitic rocks are widely distributed on that conti- 

 nent. Their great eruptive masses are of Azoic and Paheozoic age. The rocks 

 have almost throughout granitic texture, though the distinguishing features of 

 porphyritic rocks are proper to some subordinate varieties of diorite and diabase. 

 The eruptive activity exhibited in the granitic era gradually relaxed. It appears 

 to have been insignificant, though it was by no means extinct, in the latter part 

 of the Devonian and the first part of the Carboniferous periods. But it recom- 

 menced about the time of the deposition of the coal measures, thence increased in in- 

 tensity, and was most violent in the Permian age, though it was, during all that 

 time, much more limited in extent than it had been during the granitic era. The mid- 

 dle portions of Germany were then its principal theater, until it changed, in the com- 

 mencement of the Triassic age, to the southern slope of the Alps and Carpathians. 

 The rocks produced during this era possess, almost exclusively, porphyritic texture. 

 The Jurassic, Cretaceous, and the first part of the Tertiary ages, were distinguished 

 by an almost complete repose of eruptive action, in Europe. It was only after the 

 commencement of the Tertiary period when that violent resumption of eruptive ac- 

 tivity took place to which we have repeatedly called attention in the foregoing pages, 

 and which thereafter continued, gradually relaxing, down to our present era, the mani- 

 festations of volcanic action in which are'its last faint remnant. Vesicular inflation, 

 which is the characteristic feature of trachytic texture, is peculiar to the rocks of this 

 class ; all of them possess it more or less, though there are varieties resembling por- 

 phyritic rocks closely in aspect. A combination of the trachytic and porphyritic modes 

 of texture is of more common occurrence than either of them singly, and is indeed 

 the distinguishing feature of volcanic rocks. 



These relations of age and texture have been conclusively proved to exist in 

 Europe, and appeared to justify the conclusion, that the three classes of eruptive rocks 

 are geologically separated, and represent three successive and distinct phases of the 

 manifestation of subterranean agencies. Considering in their generality the facts ob- 

 served in other countries, they appeared to confirm these views, since granitic rocks 

 are known to be generally very ancient ; volcanic rocks to have been generated during 

 the Tertiary and Post-tertiary periods ; while, in regard to porphyritic rocks, observations 

 have been scarce, and descriptions lack distinctness, yet no relations have been recorded 

 in reference to them which would be contradictory to those observed in Europe. On the 

 western coast of North America, however, eruptive rocks exhibit some peculiar rela- 

 tions differing to some extent from those which they olfer in Europe. Positive facts 

 are known sparsely outside of California, but conditions similar to those observed in 

 that country appear to be repeated through large portions of the range of the Andes. 

 Prof. J. D. Whitney's admirable researches on the age of the metamorphic rocks of 

 the Sierra Nevada have clearly demonstrated that the granitic rocks, which partake to 



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