PETROLOGY. 169 
PROGRESSION OF TYPES. 
In a recent paper* Pirsson calls attention to a phase of the occurrence of the 
rock types in Central Montana Region, to which he applies the name of "Regional 
Progression of Types. " The idea to be expressed by this term is that in traversing 
a given comagmatic region certain types, from being sporadic, become frequent and 
then disappear, the rare types of one district or portion of a region becoming the 
common ones of other portions. As he says that "it would be a matter of interest 
to know if this progression of types is a peculiarity confined to this (Central Mon- 
tana) province and occasioned by the local distribution of magmas, or whether it 
is of more general application," and as he suggests the region of central Italy as 
one in which it is likely to be observed, it will be well to examine our region briefly 
from this point of view. 
It may be stated at the outset that a similar progression does seem to exist, 
though not as clearly cut in many ways, as seems to be the case in the Montana 
one. This is possibly due to the fact that in the Italian region the rocks are 
entirely extrusive. As instances of the kind the following may be mentioned : 
The galeral and hernical types of braccianose and the romal and saccal of 
albanose [leucitites] occur in abundance, and form the majority of the flows in the 
Hernican and Latian and the southern part of the Sabatinian Districts. To the 
south of the first of these, these types are rather common, though somewhat sporadic, 
in the Auruncan District, but are quite absent in the Campanian, the types of brac- 
cianose here being quite distinct. To the north these types are unknown in the 
Ciminian District, but reappear in the Vulsinian, especially in the southern part, 
to die out in the northern. 
The ischial and cumal types of phlegrose [augite-trachyte] are most abundant 
in the western part of the Campanian District, they (especially the cumal) are 
rather abundant but sporadic in the Auruncan District, disappear in the central 
ones, to recur as the scattered blocks of cumal phlegrose in the Ciminian. Simi- 
larly, but in a reversed direction, the arsal types of vulsinose and monzonose (vul- 
sinite) are fairly common in the northerly Vulsinian District, attain a maximum in 
the Ciminian, are lost in the three central districts, occur in the Auruncan, and are 
very rare in the Campanian. 
The types with the characteristic viterboid habit, viterbal vulsinose, ciminose, 
and vicose [leucite-trachyte], show a closely similar distribution. Occurring rarely 
in the Vulsinian District, they form the most prominent rocks of the Vico 
Volcano in the Ciminian, vanish in the central districts, and are rather common 
in the Auruncan, though they do not reappear in the Campanian. 
These are the most prominent examples, though others might be given or can 
be found by the reader by examination of the descriptions of the types, and they 
show clearly that a progression of types does exist in the Roman Region, and that 
such a phenomenon is not local and a peculiarity of the Central Montana Region, 
* L. V. Pirsson, Am. Jour. Sci., XX, 1905, p. 48. 
