Review of Recent Geological Literature. i8i 
the water-vapor. Thus, while the earlier, or ferro-magnesian, com- 
ponents may reach considerable size, the crystals of the later com- 
ponents are comparatively small. In the case of recurrent phenocrysts 
it is supposed that the most abundant components tend to set up 
centers of crystallization before their nomial periods, the rest of the 
process continuing as before. Ji. L. F. 
On the Occiiyrence, Origin, and Chetnical Composition of ChromitCr 
By J. H. Pratt. Am, J. Sci., 157, 281-286. 
The usual mode of occurrence of the chromite of North Carolina 
is in the form of rounded masses of varying proportions in the peri- 
■dotite near its contact with the inclosing gneiss. Its basic character 
is an indication of its insolubility in the peridotite magma; hence it 
would have been among the first of the minerals to solidify. Its re- 
lation to the inclosing rock confirms this view. With two exceptions, 
all analyses known show the presence of magnesium and aluminum, 
and the author concludes that chromite is really a combination of the 
three isomorphous molecules, FeO.Cr20,3, MgO.CroOs, and MgCAU 
0.1. The MgO.ALCs is represented by the mineral spinel, and the dis- 
covery of MgO.Cr:;©.-; as a definite mineral may be expected. 
M. L. F. 
Sonie Rock-forming Biotites and Amphiboles. By H. W. Turner. 
Analyses by W. F. Hillebrand, H. N. Stokes, and William Val- 
entine. Am. J. Sci., 157, 294-298. 
The paper includes descriptions and very complete analyses of bio- 
tite from granite, quartz-monzonite, and gneiss from the Sierra Ne- 
vada region, and of the amphibole of gabbro and quartz-monzonite 
from the same locality. Complete analyses of most of the rocks are 
also given. The work was executed in the laboratory of the U. S. 
Geol. Survey, the object being to furnish a basis for calculating the 
molecular composition of the rocks in question. m. l. f. 
On the Occurrence of Paleotrochis in Volcanic Rocks in Mexico. 
By Henry S. Williams. Am. J. Sci., 157, 335-336. 
More or less regularly striated biconical forms resembling the sup- 
posed coral Paleotrochis of Emmons were found by Alfred Duges in 
the volcanic rocks of an old eroded cone northeast of Guanajuato in 
the Santa Rosa mountains. The explanation is suggested that in the 
upward passage of superheated siliceous waters through the rock a 
point might be reached where a sudden loosening of the texture would 
permit a rapid upward radiation of the heat, while below the point the 
temperature would remain nearly constant for long periods. Deposition 
would take place wherever a cavity existed, but the supply of silica 
would be furnished only as it was abstracted by solidification. It is 
considered that the cavity would be increased in size by the crystalliza- 
tion, and that "the accretion-surface would increase with the length- 
ening of the cone by the separation of the apices of the cones, con- 
solidation taking place at the point of meeting of the solidified silica 
M-ith the water, and the water reaching that point slowly." M. L. F. 
