INTRODUCTION. 
The present paper embodies the results of an investigation undertaken under 
the auspices of the Carnegie Institution of Washington, and is the continuation 
and completion of some studies which were published several years ago. Since the 
first publications much additional material has been collected, and, while my collec- 
tions do not pretend to be exhaustive or complete, the specimens are so numerous 
and are regarded as so representative of the rocks of the region that they may prop- 
erly serve as a basis for some general discussion. 
One of the objects of the paper is the detailed description of the many rare rock 
types characterized by the presence of leucite, for which the Italian volcanoes are 
so famous, and most of which are very inadequately known. It was thought also 
that a careful chemical study of these, as well as of the accompanying non-leucitic 
rocks, might throw some light on the magmatic and physical conditions which are 
involved in and control the formation of this rare and interesting mineral. 
Another aim was the description and discussion of the characters of the region 
as a whole, as an addition to the small but growing list of petrographic provinces 
which are more or less adequately known in their petrological and petrographical 
aspects. The study of these would seem to have important bearings on some of 
the broader problems of petrology, especially the theory of differentiation and the 
question of the original homogeneity or heterogeneity of the earth. As regards 
these we are still in the observing and fact-collecting stage of the science, and it 
was thought that the description of the region from the petrological, as well as from 
the petrographical, point of view would be of some value. 
In this connection attention may be called to the introduction of the term " co- 
magmatic region " to replace the older "petrographical province." It is thought 
that the latter is unsatisfactory in that it implies only the purely petrographical 
characters, without reference to the broader petrological characters and relations (of 
which the petrographical are but one set), the knowledge of which is essential to our 
understanding of a given area of genetically related rocks as a whole. Further- 
more, the term "province" implies that the area is part of a larger one, and, while 
this may ultimately be found to be true in some instances, it would seem better to 
employ some term which does not have this connotation and which does not postu- 
late in the term itself any relations to others. 
It may be objected to the word "comagmatic" that it begs the question of 
common genetic origin for the rocks of a given region. But the word is used merely 
with the idea that the magmas of a certain area have characteristics in common, 
which is the idea underlying the use of "petrographical province," and whether the 
occurrence of these common characters is due to processes of magmatic differentia- 
tion or to other causes is not asserted or implied in the term. 
In the case of the present region, as in many others, it will be found that it is 
naturally divisible into smaller groups, around clustered or single centers of intru- 
V 
