1 6 THE ROMAN COMAGMATIC REGION. 
regarded as allowable. They are based on the descriptions of botany and zoology, 
and follow the plan of these in general, mutatis mutandis. 
These have, both in form and in language, been adopted after consultation with 
my colleagues in the quantitative system, though they may be regarded as possibly 
not definitive or final, but as an attempt at a more precise and concise statement of 
the characters of igneous rocks such as is demanded by the new system of classifi- 
cation for the establishing of types. Many of the terms used in these formal 
descriptions are some which have recently been adopted by ourselves, and which will 
be published by us elsewhere in a paper setting forth some additions to the quantita- 
tive system of classification. Most of these will be self-explanatory. 
The general plan on which these formal descriptions are arranged is as follows: 
The megascopic characters, or those determinable in the field, will be given first; 
the more general, or those which are discernible at a greater distance from the eye, 
being stated before the more particular. If phenocrysts are present, they will be 
named and briefly described megascopically in the order of their abundance. The 
statement of the specific gravity of the analyzed specimen belongs in this part of the 
description. 
Next follows the description of the characters discernible only in the thin sec- 
tion the microscopic characters. Here also the more general features of texture 
as being the more readily discernible, come first, in the order of characters pertain- 
ing to crystallinity, granularity, and fabric. Then follows a succinct statement of 
the mode, the minerals present being given in the order of abundance, and, if the 
rock is porphyritic, the phenocrystic minerals preceding those which belong to the 
groundmass. After this come the descriptions of the several minerals, given in the 
order of their statement in the norm, though the alferric minerals, which do not 
belong to the norm, are given in the order, augite, hornblende, biotite, garnet, and 
precede the femic minerals. The characters of the several minerals are stated in 
the order of their importance, their relative amount in the rock (by weight, not by 
volume), their size, crystal development, shape, twinning phenomena, color, inclu- 
sions, and arrangement. A detailed statement of the optical and other characters, 
as extinction angles, pleochroism, zonal variation, etc., will not be attempted, nor 
are data given as to the order of formation, though these last may well be considered 
to have a place in the formal description. 
It is, of course, understood that the figures expressing relative amounts and 
sizes, which have been arrived at by study under the microscope, are averages merely, 
and that other rocks may depart from one or more of them within reasonable 
limits and still be considered as belonging to the same type. 
These formal descriptions are pen pictures of the rocks skeletons which the 
imagination of the reader must clothe with reality, but which furnish in concrete 
form the absolute and relative characters with which other investigators may com- 
pare other rocks in the future. They are intended primarily for reference, sup- 
plementing and stating the facts of the ordinary descriptions. 
