PETROGRAPHY. 
Introduction. 
The present section will be devoted to the purely petrographic characters, 
that is, to the descriptions of the rocks themselves, including their magmatic or 
chemical, modal or mineralogical, and textural characters. While some of the rocks 
dealt with are well known to all petrographers, or have been already described 
according to the prevailing systems, others are here discussed, at least in detail, for the 
first time. But in order to secure uniformity, all the types which have come under 
my observation will be described, irrespective of whether they have previously 
appeared in literature or not. This is essential, because the older descriptions, based 
as they are on primarily qualitative principles of classification, are inadequate for 
the description and discussion of the region according to the newer system. Fur- 
thermore, one of the objects of this paper being the illustration of the application 
of the recently proposed quantitative system, the more rocks that are described, 
especially if of well-known types, the better will the paper serve its purpose. The 
tuffs of the region will not be discussed. 
As the specimens examined and the rock types to be described are very numer- 
ous, the descriptions will be as brief as may be consistent with their purpose. Minute 
details and certain mineral and textural peculiarities, such as the extinction angles 
of the feldspars and other minerals, schemes ofp leochroism, the details of zonal 
structure and of the arrangement and character of inclusions, etc., will either be 
omitted or only briefly alluded to. While undoubtedly of great value, such minutiae 
belong rather to the descriptions of single specimens, and considerations of space 
and time would, in any case, lead to their omission here. Direct reference to previous 
work or observations will not often be made, but further details may be looked 
for in the references given in the bibliography. 
On the other hand, the descriptions will be much more quantitative than has 
hitherto been attempted, in accordance with the fundamental principles of the 
classification and nomenclature adopted here. The necessity of this in the prem- 
ises is apparent, but the somewhat laborious and time-consuming work involved 
by this method of treatment, as well as the difficulty of applying microscopical 
measurements to rocks of such fine grain as are many of these, have limited their 
application, at least in an exact way, to those rock specimens which were analyzed 
chemically and which serve as types. The others have been classified by reference 
to and comparison with these type specimens, supplemented in some necessary 
cases by measurements under the microscope. On this account the reference of 
some of the unanalyzed rocks to certain classificatory positions, or their classifica- 
tion, is not altogether certain. It was found, however, that practice in the compari- 
son of rocks from the quantitative point of view, and the gradual assumption of a 
mental attitude of regarding them quantitatively, made the discrimination and 
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