PETROLOGY. 145 
evident from the geological descriptions of the different districts, by which it is seen 
that the outer districts the Vulsinian, Ciminian, and Sabatinian on the north, and 
the Campanian and Auruncan on the south are of complex volcanic structure, and 
for the most part made up of the products from several vents, while of the inner ones, 
the Latian consists of but a single volcano, or rather the successive sommas and 
cones around a single vent, and the Hernican of a number of very small and simple 
cones. The complexity of the magmas will be discussed later. 
In this connection it is interesting to note that the volcanoes of the Tuscan 
Region, enumerated on p. i, are all of a simple type and that their eruptive rocks 
are in each case extremely uniform, the different types of each being chiefly dis- 
tinguished by their textural characters.* Another interesting fact in connection 
with these volcanoes is that dikes are quite numerous, although they are so rare 
at the Roman ones. 
A second factor which may be significant of deeper relations is that the more 
complex volcanoes are very close to each other, and have been often in a state of 
activity contemporaneously, while the simpler central volcanoes are farther apart, 
both from each other and from the next outer complex ones. 
As the occurrence of the rocks is so uniformly that of flows or tuffs, there is 
little opportunity to observe any relations between the characters of the rocks or 
the magmas and the geologic occurrence. So far as my knowledge goes, there is 
no constant or evident relation between the two. Magmas of the most diverse 
kinds have poured forth in both small and massive flows and have assumed the 
most diverse types, sometimes highly porphyritic and sometimes aphyric, some- 
times leucitic and sometimes free from this mineral. The tuffs, likewise, are of 
very diverse petrographic characters, but as little attention was paid to them by 
me no generalization can be attempted here. 
Chemical Characters. 
The chemical characters of the Roman Region may be studied in the annexed 
table, where are collected the superior analyses which have been presented in the 
preceding pages, those heretofore unpublished except in the collection of analyses 
being indicated by an asterisk. Although they do not include every type found 
in the region, nor representatives in each district of all the types which occur in it, 
they are so numerous and cover such a wide range that they may be held to be 
representative of the whole and a satisfactory basis for generalizations. 
The chemical characters of any region manifest themselves in two ways. They 
may be constant, or practically constant, throughout the region, and consist in the 
prominence or predominance of certain constituents and the comparative subor- 
dination of others. Thus some regions, like those of Christiania and Madagascar, 
are characterized by the abundance of alkalis relative to lime, while others, like that 
of eastern Canada, show lime largely preponderating over the alkalis. Or, the 
* Cf. De Stefani, Boll. Soc. Geol. ltd., X, 1891, p. 550; also Washington, Jour. Geol., V, 1897, p. 349. 
