PETROGRAPHY. 61 
be regarded as the more reliable, both on the score of freedom from this error and 
on that of completeness. 
Compared with the norm the only divergence of note is that involved in the 
formation of biotite, which takes up the normative olivine (used as the basis for cal- 
culating the biotite) and a little of the orthoclase, as well as the transfer of a very 
small portion of the normative anorthite and magnetite to form the augite. But 
these readjustments are of slight importance and their effect is negligible, so that 
the mode may be considered normative and the rock may be described as normative 
salphyro-vulsinose-ciminose. 
Occurrence. Typical arsal vulsinose-ciminose is met with especially in the 
Ciminian District, to the west and southwest of Monte Vico, where it forms extensive 
flows. The most prominent localities are in the neighborhood of Vetralla and San 
Giovanni di Bieda, where it is quarried extensively. It is also probable that the 
type occurs in the northern part of the Vulsinian District and possibly in the Aurun- 
can, but of this there can be no certainty in the absence of specimens and the meager- 
ness of the published descriptions. 
Rocks which belong rather to ciminose-vulsinose or to vulsinose, and of this 
type, are abundant about the Cimino Volcano in the Ciminian District, as at Monte 
Pallanzana and Monte Valentino, where they form sometimes lava flows, some- 
times flow-breccias, and again tuffs. These are the peperite des hauteurs of Sabatini 
and the trachite-andesitico a grossi sanidini of Mercalli, already mentioned in connec- 
tion with sorianal harzose. In these rocks the orthoclase phenocrysts sometimes 
attain a large size, up to 5 cm. in length. The flow from the Vico Volcano at Massa 
di San Sisto, southwest of Viterbo, in the Ciminian District, and flows near San 
Lorenzo in the northern part of the Vulsinian District, also seem to belong in vul- 
sinose rather than in ciminose. As the specimens of these were not very fresh no 
analyses of them were made. 
Name. The subrang name, ciminose, like the older ciminite, is derived from 
that of the Ciminian District in which this magma is very common. The type 
name, arsal, is derived from the flow of L'Arso, on the island of Ischia, as the mode 
and texture and the habit of the rocks described above much resemble those of the 
arsal monzonose from the Campanian locality. At the same time, it must be said 
that the Arso rock differs from those from the Ciminian and Vulsinian Districts in 
the greater conspicuousness of the feldspar phenocrysts as well as in the presence 
of accessory olivine phenocrysts. On this account it might be well to distinguish 
between them and erect two types instead of one, restricting the term "arsal" to 
rocks identical with the Arso lava and calling those with less conspicuous feldspars 
and with very small amounts of biotite and no olivine, vetrallal ciminose, vulsinose, 
or monzonose, as the case may be. But it would seem unwise at the present time to 
increase the already large number of types described here, and the recognition of 
such rather unimportant differences may well be left to the future if it be then deemed 
advisable. 
As regards the position of the type in the prevailing systems of classification, 
the remarks in connection with bolsenal vulsinose (p. 33) will apply here. Placed 
