VI INTRODUCTION. 
sion or eruption. For these the term "district" is used, the individual points from 
which the igneous rocks were extruded being called "centers." 
Advantage was also taken of this opportunity to illustrate some of the prac- 
tical applications to the study of rocks of the principles of the new classification and 
nomenclature, and to test their value in petrological investigations. Such an expo- 
sition was especially needed in the matter of types and the use of the proposed rock 
names. In recent literature a number of petrographers have employed the classi- 
fication so far as to give the magmatic position of the various rocks dealt with. But 
the system, to be a practical one, as it is intended to be, must go farther than this, 
and its rock names and other parts of its nomenclature must be capable of use in 
descriptive literature, just as are those of the prevailing systems. 
No apology is therefore offered for the language in which part of the paper is 
couched, nor for the unfamiliar terms employed throughout. On the contrary, it 
is hoped that their use here will aid petrographers to a better understanding of the 
quantitative system, and that the paper will serve in some sort as a working model 
of its methods and applications. Some of the terms which are not to be found in 
the original publication of the quantitative system are the result of subsequent con- 
ferences and discussions between the joint authors of that, and they, as well as fur- 
ther additions, are expected to appear in the Journal oj Geology (vol. xiv, 1906). 
The whole region whose rocks are the present objects of study is being investi- 
gated with enthusiasm, especially from the geological and structural points of view, by 
many able Italian geologists, whose published results have been made use of largely. 
Some of the more important of these publications, with those of foreign observers, 
will be found in the bibliographical list given later. With comparatively limited 
opportunities to make the many detailed observations in the field which are needed 
to elucidate all the strictly geological questions involved in the study of the region, 
I can not venture to treat these adequately on the basis of my own knowledge and 
must leave their discussion to the geologists of the country, who have shown such 
zeal in the study of the many interesting problems which their favored peninsula 
presents. I can only express the hope that my Italian confreres will not deem me 
an unwelcome intruder into their proper sphere of investigation, but will consider 
the efforts made here to solve some of the special problems as evidence of the deep 
interest taken in the geology of their country, whose men of science have always 
shown themselves most hospitable to those from abroad. 
It is a great pleasure to express my thanks to the Trustees of the Carnegie 
Institution of Washington, who have so generously aided me in these investiga- 
tions, and have made it possible to bring to completion a long-cherished plan. To 
my colleagues in the quantitative classification, Doctor Cross, Professor Iddings, and 
Professor Pirsson, I am deeply indebted for much kind interest, advice, and aid in 
many ways. As one of the objects of this paper is to serve as an example of the 
practical application of our ideas, they have been frequently consulted, part of the 
manuscript has been examined and commented on by them, and the general form 
in which the descriptions of the types are cast is the result of our joint discussions. 
