Classification of Rocks. — Sptirr, 229 
trachytoid. The first of these terms corresponds in the present 
classification to the granular, hypidiomorphic or allotriomor- 
phic. The second corresponds to the porphyritic division with 
coarse groundmass, and the third to the porphyritic with a 
fine-grained groundmass. In addition to these three struc- 
tures aplyte has come into very general use as a structural 
term and so this fourth division is recognized in the present 
classification. The glassy structure is also added. The words 
aplyte and porphyry are purely structural and all mineralogic 
significance has been dropped. The word porphyryte has 
been abandoned on account of its mineralogic significance and 
according to the usage of American petrographers, as set forth 
in a circular of the United States Geological Survey, May 24. 
1898. Usage having fixed a definite name to the volcanic 
rocks, this usage has been retained and elaborated instead ot 
the name being supplanted by the group name with an appro- 
priate suffix, as it should be in a perfect classification. Proba- 
bly still other structural divisions will be useful and should be 
designated by appropriate suffixes. 
III. Definition of New Terms Proposed. 
Alaskyte group. — The general plan of the classification of 
feldspar rocks being explained, it remains only to treat of a 
mineralogic group now proposed for the first time, that of 
alaskytes. This group name covers many rocks which have 
been described as granite, rhyolyte, aplyte, elvan, granulyte, 
euryte, granitel, semi-granite, etc. On the other hand, not all 
the rocks described under these names belong to the alaskyte 
group, but many would belong to other mineralogic groups. 
The alaskyte group is a distinct mineral group, consisting of 
quartz and alkali feldspar only, and in this sense it is quite 
new and so a new name is given. The most common name for 
certain rocks of this kind is aplyte, defined by Rosenbusch* 
as consisting essentially of quartz and a prevailing potash feld- 
spar, and as having a characteristic fine-grained and panidio- 
morphic structure. Departing, however, from this mineralo- 
gic-structural definition, he uses the term in a purely structural 
sense for all dike rocks having a fine-grained panidiomorphic 
♦Mikroskopische Physiographic der Massigen Gesteinc, Stuttgart, 
1896, p. 459. 
