OF VOLCANIC ROCKS. 39 



closer resemblance to each other than the silicious rocks. The cause must be, either 

 that the conditions of the mass before cooling were less varied with basic than with 

 silicious rocks, or that the differences were alike, but manifested themselves less con- 

 spicuously in the character of the solidified rocks ; the latter cause is the most probable 

 of the two, since even our cotemporaneous basaltic and andesitic lavas offer but a small 

 number of varieties comparing with those of rhyolite or trachyte, although it is not 

 likely that there is any more variety among the circumstances modifying the latter than 

 among those which are acting upon the former. It is chiefly the similarity in texture 

 which has occasioned the general application of such names as " trap," " greenstone," 

 "aphanite," and others, for basic rocks of all ages, while those containing a large 

 amount of silica have scarcely ever been similarly confounded. We may, however, note 

 this difference among the former, that granitic texture — that is. a crystalline aggrega- 

 tion of the component minerals — is peculiar to those basic rocks associated with granite, 

 while the vesicular or trachytic texture is only proper to volcanic rocks. Occasionallv, 

 though very rarely, rocks of granitic texture are geologically associated with porphy- 

 ritic rocks, as is the case near Predazzo, in southern Tyrol. 



5th. Geological age : granitic rocks being generally the most ancient in origin, 

 volcanic rocks the most recent, while those of porphyritic structure are intermediate in 

 age between both. 



6th. Other geological conditions resulting from the correlation of the different 

 foregoing principles, the outlines of which I will attempt to trace in the following 

 pages. 



Correlation of Age and Texture. 



It is scarcely possible to treat of one of the foregoing principles in its appli- 

 cation to eruptive rocks singly, without constantly encountering points of intimate 

 connection with others, so thoroughly are they intertwined and mutually dependent. 

 It is the object of petrology to develop these correlations. We contemplate them 

 here only in their purely geological bearings, in order to try to establish the true rela- 

 tion of volcanic rocks to their ancient predecessors. We have, while occupied with 

 these considerations, constantly to keep in view how few arc the observations upon 

 the strength of which we have to base conclusions. The area on the globe whose 

 special geology has been made the object of investigation, though extending every 

 year, is still very limited ; and even in the best explored countries, little attention 

 has, in most instances, been paid to the distinction of eruptive rocks. Observations in 

 regard to them are abundant in some parts of Europe, fortunately in such countries 

 as are especially capable of giving a clue to their general knowledge. Distinct 

 conclusions may be arrived at in regard to portions of that continent, but little scope 

 is afforded for giving them latitude by comparison with the relations presented in other 

 countries. We have, therefore, to keep well separate in general petrology, those posi- 

 tive conclusions which are founded upon sufficient observations, and are applicable to 

 the relations in those countries where the latter were made, and the realm of theories, 

 which are arrived at, partly by the generalization of those conclusions, and partly by 

 making deductions from hypothetical suppositions; because, the premises being founded 



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