23-29 
174 THE ROMAN COMAGMATIC REGION. 
Calculating the average on the bases given above, and neglecting water and 
the minor constituents except TiO 2 and P 2 O S , the following results were obtained: 
Composition of the Average Magma. Norm o) the Average Magma. 
SiO 2 53-42 0.890 Or 48-93) 
A1 2 O 3 18.29 .179 Ab 4.72^65.60^ 
Fe 2 O 3 2.94 .018 An n-95) [-76.68 
FeO 3.44 .048 Ne 11.08 11.08) 
MgO 3.10 .078 Di 14.24) 6g 
CaO 6.37 .114 01 2.6o$ 10 ' 84 
Na 2 O 2.96 .048 Mt 4-18? f t 
K 2 8.31 .088 II 1.52) 5 ' 
TiO 2 0.82 .010 Ap 0.75 0.75 
P 2 O S 0.35 .002 
99-97 
IOO.OO 
According to this calculation the average or original magma falls almost 
exactly at the center point of dosalane, is fairly well within the lendofelic order nor- 
gare, though somewhat close to the border of the perfelic germanare, is almost 
exactly at the center point of the domalkalic rang essexase, and is well within the 
borders of the dopotassic subrang vicose. It is thus a vicose (II. 6. 2. 2), somewhat 
approaching a ciminose (II. 5. 2. 2), though not sufficiently closely to be called a 
ciminose-vicose. Its general composition is therefore that of two of the most abun- 
dant subrangs, and it is clear that the influences of the others which were introduced 
into the calculation have largely counterbalanced each other, the persalanes the 
salfemanes, the perfelic orders those which are more lenic, and the peralkalic rangs 
those which are alkalicalcic, while the sodipotassic subrangs reduce slightly the 
markedly dopotassic character of the others. 
None of the rocks which were analyzed have exactly the same composition, 
though the analyses of bagnoreal ciminose and viterbal vicose most resemble it in 
general features. In the former, however, the silica is a little higher (the order 
being perfelic on this account), and the bivalent oxides are lower, while in the 
vicose analyses alumina is a little higher and potash considerably so. 
When the position of this average analysis is referred to the diagram given 
farther on (p. 184), it is seen that it falls almost exactly in the middle of the gap be- 
C " f~\ 
tween SiO 2 = 52 .40, and the critical group, with a ratio of *= 10. n, or almost 
J\- 2 (J 
exactly the average one. It would seem from these facts that the composition as 
calculated must approximate very closely to the true average of the effusive rocks 
so far as they are known. 
It is worthy of remark that rocks belonging to vicose and ciminose, or which 
approach the average in chemical composition, occur only in those districts where 
there is the greatest variety of magmas, namely the Vulsinian, Ciminian, Saba- 
tinian, and Auruncan, and which, furthermore, we have reason to believe are the 
oldest. They seem to be entirely wanting in those districts where the rocks are 
more uniform in character, the Latian and the Hernican, as well as in the Cam- 
