ITALIA N PE TR OL O GICA L SKE TCHES 5 5 5 



olivine. Tlicsc lie in a very glassy groundmass containing fluid- 

 ally arranged plagioclasc laths and augite microlites. Neither 

 nej:)heline nor leucite could be detected. It is possible that a 

 small amount of sanidinc may be present. Klein remarks that, 

 though the appearance and the presence of olivine would sug- 

 gest a basalt, yet that the analysis (No. 7, page 565) does not 

 agree with this determination, and that consequently it must be 

 called an oli\'ine-bearing augite-andesite. 



LcHcite Rocks. — In our classification of these rocks we are 

 confronted with two difTficulties which render the bestowal of 

 correct names, in so far as this is an important matter, an affair 

 of some doubt. The first is the fact, already mentioned, that 

 the various types grade into one another mineralogically to such 

 an extent that the drawing of hard and fast lines is rendered in 

 many cases impossible. 



The second difficulty is the fact that two separate systems of 

 nomenclature have been proposed by leading petrologists for 

 some of the leucite rocks. The following are the names of Ros- 

 enbusch' and Zirkel.^ An effusive rock composed essentially of 

 leucite and orthoclase Rosenbusch calls a leucite-phonolite, 

 while Zirkel calls it a leucite-trachyte. If nepheline is added to 

 the above combination it becomes, according to the former a 

 leucitophvr, according to the latter a leucite-phonolite. It is 

 unfortunate that such a diversit)- should exist, especially in the 

 double use of the name "leucite-phonolite." It is true that it is 

 an embarrassment which seldom confronts one, since both groups 

 are of very rare occurrence. In the present case however we are, 

 so to speak, in their native land, and a decision must be come to 

 in regard to the matter. Zirkel's objections to Rosenbusch's use 

 of the term leucite-phonolite seem well grounded, since through 

 long use one connotes the presence of nepheline with the name 

 phonolite. Another objection which might be brought up is that 

 the name leucite-trachyte for this group of rocks has the prior- 

 ity, since it was used by vom Rath 3 as far back as 1867. 



'Rosenbusch, Mikr. Phys., II, 621, 1887. 



==ZiRKF.l., Lehrbuch, II, 427, 1894. 



3 VoM Rath, Zeit. d. d. Geol, Ges., XIX, 584, 1867. Cf. Rosenbusch, II, 621. 



