INTERNAL STRUCTURES OF IGNEOUS ROCKS 449 
THE RELATION OF IGNEOUS STRUCTURES TO THE FORMS OF 
IGNEOUS MASSES 
Pirsson finds the parallel arrangement of crystals and the platy 
parting of the laccoliths of the Highwood Mountains parallel to 
the roof (28), and has some evidence of a similar relation at Tri- 
pyramid Mountain (29). Iddings reports the parting and color 
banding of the Mt. Holmes ‘‘bysmalith”’ (27) parallel to the walls. 
_F1c. 9.—Sheeted structure in the Duluth gabbro evidently independent of the 
surface. Spheroidal weathering also appears. 
Rogers finds that the bands in the Cortlandt gneiss bear no definite 
relations to the borders of the magma (11). However, banding 
in the Adirondacks is of several kinds, and Miller records ‘‘a folia- 
tion that boxes the compass around the borders of the stocks”’ (30). 
The banding in lava flows and their trachytic structures is often 
recorded as parallel to the general plane of the flow. Examples 
Amer., XXVIII, 455. ‘‘Fluidal gneiss” and “‘injection gneiss,’’ as terms recently 
developed in structural geology, are probably best restricted to another type of struc- 
ture. It is detected in tracing igneous injections in bands between masses of a schist 
of other cleaved rock, or even curving in and out among rock fragments. This results 
in an alternation of the original cleaved rock (of whatever origin) and the igneous 
rock. Solution of the original rock and its metamorphism by the magma may pro- 
duce such an intimate intergrowth as to make distinctions between intrusive and 
intimate intergrowth as to make distinctions between intrusive and intruded rocks 
difficult. See Leith, Structural Geology, p. 85; and Cross, Science, XXIX, 946. 
