ITALIAN PE TR OL O GICA L SKE TCHES 3 5 9 



NOTES TO TABLE II. 



1. Bolsena, H. S. Washington anal., Jour.Geol., IV, 552, 1896. 



2. Bolsena, Vom Rath, Zeit. d. d. geol. Ges., XX, 291, 1868. 



3. Bolsena, Ricciardi anal., Klein, Neu. Jahrb. B. B., VI, 8, 1889. 



4. Vetralla, near Viterbo, H. S. Washington anal., JouR. Geol., IV, 849, 1896. 



5. San Magno, Latera, Bolsena, Ricciardi anal., Klein, loc. cit., 10. 



6. Torre Alfina, Bolsena, Ricciardi anal., Klein, loc. cit., 3. 



7. San Lorenzo, Bolsena, Ricciardi anal., Klein, loc. cit., 3. 



8. Monte Amiata, Ricciardi, Gazz. Chim. Ital., XVIII, 1888. 



9. Monte Santa Croce, Rocca Monfina, H. S. VVashington anal., JouR. Geol., 

 V, 252, 1897. 



10. Monte Santa Croce, Vom Rath, Zeit. d. d. geol. Ges., XXV, 245, 1873. 



In the last rock (9 and 10) we have an example of the 

 mutual exclusion of biotite and olivine in the Italian and other 

 trachydolerites, to which attention has already been called by 

 Rosenbusch." It is mineralogically a biotite-vulsinite, in that it 

 is free from olivine, but chemically a ciminite, the silica being 

 low and magnesia and lime high. The explanation of this pecul- 

 iarity may be looked for in the complexity of the biotite mole- 

 cule. It was pointed out by Iddings^ that, since the biotite 

 molecule may be regarded as made up of molecules of olivine 

 2(Mg, Fe) O, SiOg and feldspathoid molecules of the form (H, 

 K) 2 O. AI2O3. 2 SiOg, under conditions where the complex 

 biotite molecule would be unstable it would dissociate, resulting 

 in the formation of crystals of olivine and of a potash-alumina 

 silicate, either leucite or orthoclase, according to circumstances. 

 This explanation has also been adopted by Backstrom^ in the 

 case of the leucite-basanites of Volcanello, he considering them 

 to be the effusive forms of magmas which would appear intru- 

 sively as minettes or kersantites. The same idea has lately been 

 alluded to by Pirsson,'* who adds : "This process would of course 

 find its most natural expression in magmas rich in magnesia and 

 potash," which, it will be observed, is the character of the cim- 

 inite magma. 



' RosENBUSCH, Mikr. Phys., II, 773, 1896. 



' Iddings, Origin Igneous Rocks, Bull. Phil. Soc. Washington, XII, 166, 172, 1892. 



3Backstr6m, Geol. Foren. Stockh. Forh., XVIII, 155, 1896. 



^ PiRSsoN, Jour. Geol., IV, 687, 1896. 



