234 J.D. TRUEMAN 
Among the characteristics of this texture," according to Gruben- 
mann, are the following: 
t. Lens-like and roundish forms of the minerals, well-developed 
crystal outlines not generally being present. When crystal forms 
do occur they are generally simple. Foliation of minerals is 
frequently developed. 
2. The relative perfection of mineral form is dependent on the 
character of the minerals rather than upon their order of crystalliza- 
tion. The usual series of form development is as follows: titanite, 
rutile, hematite, ilmenite, garnet, tourmaline, staurolite, cyanite— 
epidote, zoicite—pyroxene, hornblende—magnesite, dolomite, al- 
bite, mica, chlorite, talc—calcite—quartz, plagioclase—orthoclase, 
microcline. In general the series is one of decreasing specific 
gravity or increasing molecular volume. 
3. Marked development of crystal faces which are parallel to 
planes of mineral cleavage. 
4. Characteristic mineral inclusions. In igneous rocks the 
inclusions are usually well-developed crystals which have solidified 
early. In the “crystalline schists” the later formed minerals may 
have more perfect outlines than their inclusions. 
5. General absence of zones of different composition in minerals. 
6. Holocrystalline character. 
7. Tendency toward uniformity in size of grain. 
Grubenmann’s conception of the causes which underlie the 
differences in character between the textures of igneous and meta- 
morphic rocks may be outlined as follows: He regards the forms of 
the minerals in igneous rocks as dependent largely on their order 
of crystallization, i.e., the earlier formed minerals have good crystal 
outlines while those of later development, having been compelled 
to occupy the remaining spaces, are irregular in form. In meta- 
morphic rocks, on the other hand, he considers that the crystalliza- 
tion of all the minerals has been more or less hindered by the solid 
condition of the rock and that those minerals possess the best 
developed forms which have the strongest crystallizing force. 
This ‘‘force,” Grubenmann regards as greatest in minerals which 
*Grubenmann uses the word ‘‘Strukur” in place of “texture.” “Strukur,’’ 
as defined by him, relates to the form and size of the constituents. ‘‘Textur’’ is used 
for their arrangement. 
