170 THE ROMAN COMAGMATIC REGION. 
but one which is widespread and probably to be observed elsewhere, now that 
attention has been directed to it. 
THE ROCKS OF THE TUSCAN REGION. 
In this connection a few words may be devoted to the relations of the rocks 
of the volcanoes making up the Tuscan Region to those considered in this paper. 
It has been mentioned that these volcanoes are all of rather small size and area, 
their eruptions being apparently of the domal type, and that they lie close to the 
area of the Roman, but exterior to it, on the north and west. 
It is unnecessary here to go into descriptions of the rocks, which will be found 
in several publications mentioned in the bibliography, especially in Nos. 4, 15, 43, 
62, and 65. It must suffice to say that the rocks are quite uniform in character, 
mostly toscanites or quartz-trachyandesites in the prevailing nomenclature, while 
in the quantitative classification they fall in dellenose (I. 4. 2. 2), toscanose (I. 4. 
2. 3), or amiatose (I. 4. 3. 3).* 
This general uniformity, or rather comparatively restricted range in chemical 
composition, their persalic and quardofelic characters, and the prevalence of sodi- 
potassic subrangs are features which fall in line with the general course of the dis- 
tribution of magmas in the Roman Region, where we have just noted the more 
salic, felic, and sodipotassic characters of the exterior portions as contrasted with 
the central ones. It suggests the thought that these Tuscan rocks may belong to 
the same general body of magmas as those of the Roman Region, and that they 
represent in fact the extreme exterior differentiation products. 
While this hypothetical magma tic connection is very tempting, it must be 
noted that a serious objection to it lies in the fact that the volcanoes of the Tuscan 
Region are apparently all of about Eocene age, while those of the Roman Region 
are much later, belonging to the Pliocene or even modern times. 
Whether this objection on the score of time relations is a valid one or not, we 
are scarcely in a position to judge as yet, though consideration of the very great 
time intervals involved would seem to render the objection to the hypothesis of 
more weight than the chemical and petrological evidence in its favor. At the same 
time it may be mentioned that the actual time interval between the earliest and 
the latest of the eruptions in the Roman Region, which are undoubtedly connected 
magmatically, is itself very great, so that possibly the objection raised here is not 
of such a serious character as might appear at first sight. At any rate, the hypoth- 
esis here presented may be borne in mind in further studies of the Italian volcanic 
rocks. 
Quantitative Relations. 
In any comagmatic region where the igneous rocks are all effusive it is in a 
high degree improbable that they represent, in the aggregate, the composition of 
the original magma from which they have been derived. This is so since, assuming 
* Cf. Washington, Prof. Paper, U. S. Geol. Surv. No. 14, 1003, pp. 150, 171, 185. 
