2 50 HENRYS. WASHINGTON 



with other occurrences. A scanty glassy base is also present. 

 No nepheline was detected in any of the above rocks, 



Biotite-vulsiiiite. — In the descriptions of the volcanic centers 

 which form the subject of the preceding papers we have had 

 occasion to examine certain members of a group of effusive rocks 

 which, both chemically and mineralogically, stand in a position 

 intermediate between the trachytes and the andesites. The 

 Rocca Monfina region resembles the others in furnishing — and 

 very prominently — members of this group. We find here in fact 

 effusive rocks showing the mineral combination of alkali feld- 

 spar and basic plagioclase, which approximate closely to the 

 ciminite of the Viterbo region in chemical composition. Miner- 

 alogically, however, they come closer to vulsinite of Lake Bol- 

 sena, no olivine being present, but augite and especially biotite 

 being abundant representatives of the ferromagnesian minerals. 

 So much indeed do they resemble vulsinite that they were con- 

 sidered at first to belong to this group, forming a species which 

 would be called a biotite-vulsinite, as was briefly noticed in the 

 paper on the Bolsena Region.^ A chemical analysis, however, 

 which I completed after the printing of that article and which 

 will be found on page 252, shows that this determination is not 

 quite correct. It will be seen that the silica is notably lower 

 than in the vulsinites, and furthermore that lime, magnesia and 

 iron are very much higher, while the alkalies are considerably 

 lower. Comparison with the analysis of the Viterbo ciminite 

 (which is inserted for convenience) will show that the trio are 

 chemically almost identical. We have then a rock which is chem- 

 ically a ciminite and mineralogically, a biotite-vulsinite. 



In regard to the name by which they should be called there 

 may be some doubt. From a mineralogical standpoint they 

 are obviously not ciminite, nor chemically can they strictly be 

 called vulsinite. Since, however, in the schemes of classifica- 

 tion in general use at the present time, the mineralogical com- 

 position takes precedence over the chemical, and bearing in 

 mind the unadvisability of adding new names to the already 



'Jour, of Geol., IV, pp. 551-553, 1896. 



