360 HENRY S. fVA SIZING TON 



Applying this idea to the rocks at hand it is evident that we 

 may regard the olivine-bearing ciminites as formed from rather 

 basic trachydoleritic magmas, rich in magnesia, which under 

 other conditions of solidification would have assumed the form 

 of biotite-vulsinites — thus adding another to the number of 

 cases of magmas of the same chemical composition forming on 

 solidification different mineral aggregrates. In this connection 

 and as favoring this view of the dissociation of the biotite mole- 

 cule may be mentioned the great paucity in biotite of all the 

 leucitic rocks of Italy, and, it may be added, of the purely 

 orthoclase-trachytes of Ischia. There seems to be, indeed, as 

 demanded by the theory, a mutual exclusion of biotite and leu- 

 cite. Olivine, it is true, is not very abundant in the leucitic 

 rocks, except at Vesuvius, the magnesia generally entering into 

 the pyroxene molecules, perhaps owing to the richness of the 

 magma in CaO. 



Toscanite. — The rocks belonging to this group are highly 

 porphyritic and a glassy groundmass is often met with, though 

 holocrystalline forms occur. They are characterized by the 

 presence, along with the alkali feldspar, of a plagioclase which is 

 rather more acid than in the preceding groups, generally vary- 

 ing from andesine to labradorite, though anorthite is found at 

 Monte Amiata and elsewhere. The most constant and promi- 

 nent ferromagnesian mineral is biotite, which is never wholly 

 wanting, and at some localities, as at Campiglia and Rocca 

 Strada, is the only colored constituent. In other places (Brac- 

 ciano, Cerveteri, and Tolfa) atigite or diopside is abundant along 

 with the biotite ; while again, as at Monte Amiata, hypersthene 

 replaces augite as companion to the biotite. Quartz is often 

 present, either in well-shaped crystals or as anhedra, but olivine 

 is never found. Magnetite is rare or absent. Silica is much 

 higher than in the other groups, varying from 65 to 73 per cent., 

 alumina and iron are low, magnesia and lirrre high considering 

 the acidity, and the alkalies also high, with potash higher than 

 soda. We may then regard the toscanites as acid biotite-trachy- 

 dolerites, and the rocks of Bracciano, Cerveteri, and Tolfa would 



