Site of the Ancient City of the Aurunci. 229 



volcanoes, whether we regard their geological structure, and 

 relations to other rocks, or their constitution, as determined 

 by chemical analysis. For the first element in the considera- 

 tion I would appeal to the testimony of Professor Keilhau of 

 Norway, who, by a long series of elaborate observations, has 

 at least succeeded in shewing the difficulties of identifying 

 granitic rocks with modern igneous products ; and I am happy 

 to find Sir Roderick Murchison, in his recent work on Rus- 

 sia, adding the testimony of his extended experience to the 

 soundness of those views with respect to the primogeniture 

 of granite, which were prevalent in the earlier times of geo- 

 logy. For although its occurrence superimposed on second- 

 ary and even on tertiary strata cannot be denied, yet it does 

 not therefore follow, that those extensive formations of the 

 same kind which lie below the most ancient fossiliferous 

 rocks are to be placed with them under the same category. 



We may, therefore, appeal to the production of a granitic 

 compound, as an example of a mode of igneous operation 

 differing from any which we witness at present, and brought 

 about under conditions which seem in the later periods of the 

 earth's history to be, to say the least, but of unfrequent oc- 

 currence. 



But this is not all, for by reference to the discoveries of 

 modern chemistry we shall be enabled, if I mistake not, to 

 trace a very beautiful series of transitions, by which the 

 primeval granite has been converted, first into trachyte, and 

 afterwards into the various kinds of lava, &;c., which charac- 

 terise modern volcanoes. 



On this point I will enlarge a little, as a full explanation 

 of it may tend to impart to you, as it has done to myself, 

 clearer notions as to the real nature of the products which 

 we are accustomed somewhat vaguely to designate under the 

 names of granites, trachytes, and the like. 



Ask, for instance, a geologist what he means by the term 

 granite, and he will tell you that he understands an aggre- 

 gate of three minerals, namely, quartz, mica, and felspar, in 

 a state of intimate mixture. 



But in order to learn what these several minerals really 

 are, the chemist is the fittest person to be consulted ; and from 



VOL. XLI. NO. LXXXIL— OCTOBER 1846. Q 



