6 THEN OGLNAIE OF NGL OE OGN4 
The form of the orthorhombic pyroxene determines in large 
measure the structure of the rock containing it. In the most 
purely granulitic varieties this mineral is in the small.rounded or 
subangular grains, which, with the similarly shaped diallage grains 
and those of plagioclase, together with an occasional particle of 
magnetite, make up the entire rock. The magnetite is included 
within both of the pyroxenes and the feldspar, and the pyroxenes 
‘are often also enclosed within the plagioclase. The sequence is 
Fic. 1.—Granulitic gabbro, with large grains of green hornblende and small 
rounded ones of augite in a groundmass of plagioclase that appears in ordinary light 
to be a homogenous mass. The large plates of mineral to the right are hornblende. 
No. M 1335, x 87. 
thus magnetite, pyroxene, and plagioclase, with the hypersthene 
probably older than the diallage. Figure 1 illustrates the struc- 
ture of a rock of this kind. 
A very peculiar type of structure is that resulting from the 
occurrence of the hypersthene in cellular grains and plates that 
are larger than the grains of the other constituents, and less 
rounded than the hypersthene of the more purely granulitic rocks. 
These large plates and grains are optically continuous over con- 
siderable areas, but are physically not continuous, since they are 
filled with small and large pores containing plagioclase of the 
same nature as that occurring between the hypersthene. The 
plates are very ragged along their edges and have many pro- 
longations with rounded ends. The skeleton-ike masses of 
