342 J. D. Dana on some points in Lithology. 
the more fundamental differences. As the name gabbro has 
Adirondacks, British America and other regions, sometimes 
called Hypersthenyte, and this name is so used by Zirkel. 
3. Porphyritic Structure—Porphyry naturally took the posi- 
tion of a species in the mineralogy of the ancients. But it is 
now well known, and generally admitted, that the porphyritic 
structure is largly due to conditions attending the former tem- 
perature and cooling of the rock-mass, and distinguishes only 
varieties. But still it is usual to find dioryte divided, for its 
primary subdivisions, into ordinary dioryte and dioryte-porph- 
yry; diabase into granular diabase and diabase-porphyry or 
diabase-porphyrite; felsyte into felsyte and felsyte-porphyry ; 
and so on, as if the porphyritic structure were deserving of 
first prominence in the question of division into varieties, even 
greater than mineral constitution; and sometimes it is even 
made the basis of a distinct kind of rock. 
fourths of an inch broad) and this last graduates near by into 
ordinary gneiss; and gradations from porphyrytic to ordinary 
