OF VOLCANIC ROCKS. 43 



aadesite, and basalt, we perceive that the former two were the precursors of the latter, 

 containing at the same time silica in a proportion superior to basalt. The singular 

 phenomenon of trachyte and rhyolite being ejected at a time intermediate between 

 the epochs of those rocks, is a notable deviation from the mode of succession peculiar 

 to granitic and porphyritic rocks. We will try to consider its probable causes in another 

 chapter. 



Correlation of Eruptive Rocks in regard to their Geographical Distribution. 



Another point of view offering in the contemplation of eruptive rocks, and which 

 has a close bearing to their natural system, is the correlation among them in regard to 

 their geographical distribution. This is, however, a vast subject, and volumes might 

 be written in collecting evidence for final argument of general value. At this place 

 we can give it only a cursory notice. We will consider, first, what is the mode of 

 distribution peculiar to each class of rocks, and then trace the correlations perceptible 

 among them in regard to their different modes of distribution. 



Granitic rocks are scattered widely over the globe. Wherever its surface is 

 composed of ancient sedimentary rocks, and these give evidence of disturbances of 

 some intensity, b}' the plication of their strata, we may be almost certain to find granite 

 entering into the geological composition to some extent. In the diversified structure 

 of the European continent, geological maps show the existence of granite in nearly 

 every prominent mountain-range. Considering among them the range of the Alps and 

 Carpathians, we find granite scattered over its whole extent, from Savoy to Transyl- 

 vania, particularly on the southern slope. It forms some prominent summits, but 

 occurs also in subordinate positions. The geological relations of several of these 

 places have been examined, and careful investigations made of the mineral and chem- 

 ical composition of the rocks. They have resulted in proving an individuality of 

 granite such as is peculiar to no other kind of eruptive rocks ; different granitic 

 masses are of different age, and exhibit a corresponding diversity in regard to the 

 composition of the predominating rocks. Sometimes, it is true, several neighboring 

 masses are similar in respect to their petrographical character, and bear evidence of 

 being nearly of the same age (for instance those of the Adamello, the Cirria d' Asia, 

 and Brixen), but others, next adjoining, will be found differing from them in nature 

 and in age. The length of geological time during which they have been ejected, has 

 never been established : but the period appears to have been one of immense dura- 

 tion. A similar individuality as to age and mineral character may be noticed in 

 respect to the granite of other mountain ranges on the European contiuent. Alto- 

 gether, the mode of distribution of the ancient granitic rocks, as far as they enter into 

 the structure of the surface, may be said to be in numerous small districts, which are 

 independent of each other in regard to their epoch of ejection and petrographical char- 

 acter. With reference to the latter, each separate district shows a great preponder- 

 ance of rocks belonging to one of the families of the granitic order, which are usually 

 accompanied by syenite in smaller proportion, and by some subordinate eruptions of dior- 

 ite and diabase. These districts are principally scattered along the present lines of eleva- 



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