PETROLOGY. 149 
usually true of European rocks. Zirconia, on the other hand, is always low, never 
rising above o . i per cent and sometimes being either absent or present in unweigh- 
able traces. The content of phosphorus is also low, the percentages of phosphoric 
pentoxide running from o . i to i . o, though figures higher than o . 5 are exceptional. 
Sulphur trioxide and chlorine are seldom met with in more than traces, and are 
frequently absent, though about i per cent of SO 3 is shown in one very rare type. 
Carbon dioxide seems to be uniformly absent, a somewhat remarkable fact in view 
of the generally high content in lime, and even though fresh and unaltered rocks 
were chosen for analysis. 
The oxides of the rare-earth metals, when looked for, were found in very 
small amount both in salic and femic types, and it is possible that traces of them 
are uniformly present in most of the rocks. Chromium was only observed once, 
in a rock rich in olivine, and even here in scarcely more than a trace, and was 
apparently absent in all the others. Nickel seems to be absent, except as possible 
traces in the more femic rocks, and the absence of copper in all cases where tests 
were made for it is noteworthy, as it is often supposed that this element is quite 
frequent in Italian igneous rocks,* on the basis of analyses by Specialef of lavas 
from the Hernican District and by ForstnerJ of lavas from Pantelleria. The pres- 
ence of copper in the last will be investigated shortly on material recently collected 
by the writer. 
Although for reasons explained elsewhere manganese was not estimated in 
any of the rocks, the color of the sodium carbonate fusion showed that it is always 
present. But it probably seldom rises above 0.25 nor falls below 0.05 per cent, 
and an average of about o.io per cent would be very close to the truth. Of the 
other elements the occurrence of baryta in, for this constituent, quite large amounts, 
is very noteworthy. It was found in practically every case where it was looked 
for in amounts varying from o . 06 to 0.29 per cent. Strontia was, unfortunately, 
only occasionally estimated and was always found to be in scarcely more than 
traces when present, and occasionally absent. 
Connected with these absolute characters, or rather as an extension or more 
detailed statement of some of them, are certain relations between some of the con- 
stituents which persist more or less uniformly throughout the whole series. The 
first which may be mentioned is the general deficiency of the magmas in silica. 
In only two or three cases is there an excess of silica over the bases, that is, more 
than sufficient to form the most highly silicated minerals. In the great majority 
of the rocks, even in those which are perfelic, there is so little of this that the 
less highly silicated minerals, as leucite, nephelite, olivine, or melilite, are formed, 
either in the norm or in the mode, and usually in both. 
A second important relation is the dominance of potash over soda (molecu- 
larly), which is very marked, and which is shown in the systematic classification by 
* Cf. J. H. L. Vogt, Zeiis. prakt. Geol., 1898, p. 317. 
t S. Speciale, Boll. Com. Geol. Ital., 1879, P- 32- 
% H. Forstner, Zeits. Kryst., VIII, 1883, pp. 173, 179, 182. 
