The Deteniiiiiation of Feldspars. — Spurr. 383 
parallel to the base, but varies from it according- to the differ- 
ent species, by an angle which must be added or subtracted 
from the reading, according to the case, to get the true angle 
with the base. 
If the bisectrix is negative, determine the angle between the 
trace of the optic plane and that of the brachypinacoid. The 
latter may be represented in a section either by a crystal edge, 
by a brachypinacoidal cleavage or by twinning planes of the 
albite or Carlsbad laws. 
1 
In either case find the angles corresponding to the species 
in the accompanying tables. 
REVIEW OF RECENT GEOLOGICAL 
LITERATURE. 
State Geological Survey of North Dakota, Second Biennial Report. 
Frank A. Wilder, State Geologist. Pages 262; with 39 plates, 21 
figures in the text, and two folded maps. Bismarck, December, 1902. 
This survey was begun in the year 1899, under the direction of pro- 
fessor E. J. Babcock, professor of chemistry and dean of the College 
of Mines in the University of North Dakota, at Grand Forks, with a 
very small appropriation from the state. His report on the general 
topographic and geologic features of the state, and on its clays of eco- 
nomic value, lignite, and water resources, in 103 pages, was the first 
biennial report of this survey, published in the early part of 1901. With 
the appointment of Mr. Wilder as professor of geology In tlie univer- 
sity, the direction of the state survey was transferred to him ; and, in 
co-operation with the U. S. Geological Survey, he has been enabled to 
enlarge the scope of this work. 
About two-thirds of this report are descriptions and discussions of 
the valuable lignite coal deposits which occupy the western half of the 
state, with the Turtle mountains as- an outlying area farther east. Pro- 
fessor Wilder was assisted by Mr. L. H. Wood in this part of his work. 
The lignite beds, in their workable development, are restricted to the 
Laramie series of clays and sandstones; and the seams of lignite vary 
in thickness from an inch to forty feet. Many seams are often found 
in the same vertical section, and some of them are traced five or six 
miles, or more, as in the bluffs of the Little Missouri river; but gener- 
ally they vary greatly in thickness within such distances, and Trequenitly 
are of much less extent, tbiiniing out entirely, and giving place to new 
seams a little higher or lower. Along the Northern Pacific railway 
in the region of Dickinson. Medora, and Sentinel Butte, and thence for 
