ITALIAN PETROLOGICAL SKETCHES 3^5 



facts we are led to believe that the parent magma of the Italian 

 rocks was not only more acid than at the Yellowstone, but 

 was preeminently a potash magma, which is certainly not true 

 of the other. It may be added that the occurrence of two such 

 well-marked and generally similar series of orthoclase-plagio- 

 clase rocks at widely distant localities forms a strong argument 

 for the recognition of such types in accordance with Brogger's 

 views. 



Correlation of the tracJiydolcrites. — Before passing on to the 

 leucitic rocks it will be as well to examine the relationships of 

 the trachydolerites and see what their position is in regard to 

 other rocks of our classification. Brogger has so ably presented 

 the arguments for the recognition of intermediate types that 

 nothing need be added on that score, since all the arguments 

 advanced by him for the recognition of the monzonites as an 

 independent group of the same rank as the syenites and diorites 

 apply with equal force to the trachydolerites and trachy- 

 andesites. The chief objection which may be brought against 

 such ideas is that their acceptance would tend to "over- 

 burden petrological literature with new names." This is 

 undoubtedly true to some extent, and many, perhaps most, of 

 these names might be but temporary in their use, but they would 

 yet fulfill their legitimate object of enabling us to comprise a 

 given set of characters in one word, make our ideas of the vari- 

 ous rock types more clear and precise, and thus lead us toward 

 the solution of that vexed question — a rational and generally 

 accepted classification of rocks, such as is found in the organic 

 sciences.' The law of the survival of the fittest would hold good 

 here as in animate nature. The needless names and types will be 

 gradually discarded, and on what is left we may build a nomen- 

 clature of which the terms will be concordant both with each 

 other and with the facts of nature, whenever the broad principle 

 underlying the relationships of rocks shall have been discovered, 



' In these, by the way, names are far more numerous than in petrology, and it ma}' 

 be added that it would seem that the advantage of having well-defined types would 

 outweigh the disadvantage of putting a little greater tax on our memories. 



