242 J. D. TRUEMAN 
growth of unusually elongated grains parallel to the direction of 
least pressure. 
t. The measurements previously given indicate that biotites 
which have crystallized slowly from igneous melts are not platy 
in the majority of cases, and that before being interfered in crys- 
tallization by other minerals they were not far from cubical in 
form. ‘The orientation of such grains in a fluid magma must then 
be almost impossible. 
2. The extremely irregular outlines of biotites in normal igneous 
rocks suggest that biotites do not generally crystallize much in 
advance of the quartz and the feldspar so that completely formed 
biotite grains cannot be considered to have been ever floating 
loosely in a fluid magma. 
3. Measurements made of biotite grains in a gneiss, which is 
considered by the writer from field evidences to possess original 
banding, gave values of from 2.5 to 5.0 for the ratio of the length 
in the direction of mineral cleavage to that across it. As similar 
tests on igneous rocks usually gave values from 1 to 1.7 it can be 
seen that the elongation of biotite is in this case decidedly greater 
than that characteristic of deep-seated igneous rocks which have 
solidified under quiet conditions. 
4. There seems no reason to think that the biotites of primary 
gneisses have crystallized under viscous or strongly supersaturated 
conditions since such rocks are generally of deep-seated origin and 
probably. contained abundant water when in the fluid state: 
- Elongated feldspar grains have been described as having been 
formed during original crystallization in the Twilight" gneissoid 
granite of Colorado. In this rock the feldspars in the more foliated 
portions occur as elongated anhedra, while in places where the 
rock is more massive they possess crystal outlines. In the former 
case the direction of elongation seems to be unrelated to the 
crystallographic directions of the mineral. 
The elongation of quartz has also been observed by the writer 
in thin sections of the biotite: gneiss previously mentioned as 
exhibiting primary banding. | 
One who grants that pressure can be sufficiently active during 
«W. Cross, E. Howe, J. D. Irving, W. H. Emmons, U.S.G.S. Folio 131 (1905), 7- 
