212 Classificution of Massive Rocks. 
(a 1) hornblende gabbro, in which diallage is partly 
replaced by hornblende, and 
(a 2) hyperite, containing a little olivine and some 
orthorhombic pyroxene. 
(B) Olivine gabbro, olivine-bearing. 
2. Norires, which contain an orthorhombic augite as the principal 
pyroxenic component. 
The norites are divided, according as to whether 
they contain olivine or not, into 
(A) norite, and 
(B) olivine norite. 
F. THE DIABASES. 
The diabases are composed essentially of plagioclase and augite, 
with or without olivine and quartz. 
They form a well-marked class among the intrusive rocks, which 
differs in many respects from the other intrusives, and approaches 
very near in characteristics to some of the effusives, This is due 
principally to the fact that the diabases occur as dykes and intruded 
layers between sedimentary beds, and thus tend to assume in some 
degree the structure possessed by sheets which have cooled on the 
surface under atmospheric pressure alone. They are frequently 
accompanied by tufas, and they often possess amygdaloidal upper 
surfaces. Since, however, the pressure under which they were 
formed was much greater than that under which the volcanic rocks 
were produced, and, as we may suppose, their cooling much more — 
gradual, the diabases are holocrystalline and hypidiomorphic- 
granular, as distinguished from the hypocrystalline and porphyritie 
structures of the members of the effusive class. Nevertheless, the 
tendency of the plagioclase to assume idiomorphic forms is so strong 
that an approach to the porphyritic structure is noticeable in many 
diabases, As among the gabbros, the first differentiation of the 
diabases is dependent upon the presence or absence of olivine as an 
essential constituent. 
1. Drapases are olivine-free combinations of plagioclase and augite, 
usually with a little hornblende and mica. 
(A) diabase proper contains no quartz. 
(B) quartz diabase contains quartz as a primary compo- 
‘nent, including 
