PETROGRAPHY. 15 
reserve them for the more detailed and special investigations. From examination 
of the subsequent pages, and when it is considered that the rocks of any one region 
have certain chemical characters in common, it is fairly clear that grads and subgrads 
can be dispensed with in great part for general purposes, and that the use of type 
adjectives will replace them advantageously to a large extent. 
In each subrang the general order will be that those types whose modes are 
most normative will be first described, followed by those which are abnormative, 
in each case the most coarsely granular, or those with the largest and most abundant 
phenocrysts, preceding the less porphyritic or aphyric ones. The hyaline types 
will be the last to be described. 
In a number of cases the rock habits will be named that is, those more gen- 
eral features of mode and texture or both in which rocks may resemble each other 
without belonging to the same type. The habits named, however, are only a few, 
and are those which seem to be most common to, or characteristic of, the rocks of 
the region. 
In the statement of the norm one modification from the original has been 
introduced, the result of a joint discussion of the subject on the part of all the authors 
of the quantitative system. This has to do with the normative minerals sodalite 
and noselite. Without going into details here, it was thought advisable to modify 
these to the extent of splitting them up and stating in the norm the amounts of 
halite (NaCl) with the symbol HI, and of thenardite (Na 2 SO 4 ) with the symbol 
Th, instead of sodalite and noselite. The soda which was previously combined 
with the sodium chloride and sulphate remains with the rest in calculating the norm, 
and, if necessary, is distributed between albite and nephelite in the usual way. 
In some cases a change in the position of the rock as regards the order is thus 
brought about, but this will happen only when the amounts of chlorine or sulphuric 
anhydride are large; and, as a matter of fact, it was found that in the present series 
the systematic position of the rock was not altered, nor the relative amounts of 
feldspar and lenad changed in the slightest, in any of the few cases in which the 
sodalite minerals are present. An advantage of this method of procedure is that it 
minimizes the influence of the small amounts of Cl and SO 3 usually found, which 
is very great if they bind up in the norm a much greater amount of soda and silica. 
This matter will be discussed in a forthcoming joint paper, along with other modi- 
fications of and additions to the system. 
THE FORMAL DESCRIPTIONS. 
Following the descriptions of the different types, which are couched in the usual 
petrographical terms, except such as belong to the phraseology of the quantitative 
system, are what may be called formal descriptions of the types. These aim to give 
a concise but complete description oi the type, both qualitative and quantitative, 
which may be regarded as standard, and from which one may judge whether 
another rock belongs to the same type or not, certain narrow departures being 
