442 The American Naturalist. [May, 
after prolonged action of the etching agent. It is found that 
the acid acts very unequally on different parts of the sphere, 
corresponding to the different crystallographic faces of the 
crystal from which it was cut, but equally with reference to 
the system of hexagonal axes. In this way the tetartohedral 
symmetry of the mineral is strikingly revealed. The action is 
greatest at the two extremities of the vertical axes, while at 
the ends of the lateral axes it seems to be almost nil. As a 
final result of the action of the etching agent the sphere is re- 
duced to a lenticular body with a triangular cross section, with 
the three angles of the triangle at extremities of the lateral 
axes. — A comparison of the shapes and positions of the etched 
figures produced on halite and sylvite upon their exposure to 
moist air has been made by Brauns.' Those in rock salt are 
usually bounded by the planes of a tetratis hexahedron, which 
may vary in formula between <» O i and oo O". Occasionally 
a depression bounded by the planes I O is 3 observed. In 
both cases the position of the figures on the faces etched are 
such that they possess the same planes of symmetry as does 
the face upon which they are. On sylvite, on the other hand, 
the depressions have no planes of symmetry in common with 
those of the crystal face. Halite is therefore regarded as hol- 
ohedral, while sylvite is gyroidally hemihedral. The same 
age pieces of rock salt, whose twinning plane is 20 O. 
New Books and Pamphlets. — " Les Mineraux des 
Roches '" is the first French book that treats of optical prop- 
erties of minerals in a way to be of use to students in the 
study of thin sections of rocks. As the authors state in their 
preface, the new book is a natural complement to Fouque and 
Levy's " Mineralogie Micrographique." In the first part the 
author (Levy) discusses the application of the principles of 
optical mineralogy to the study of minerals in thin sections of 
rocks. The methods made use of in this discussion are some- 
what new to petrography, as they are based more upon math- 
ematical considerations than is usual. The fundaments of 
crystallography and of optical mineralogy are presupposed, as 
is also a knowledge of spherical geometry and trigonometry. 
After deducing the mathematical relations of the optical axes 
and bisectrices, the curves of extinction in the principal zones 
" Neaes Jahrb. fur Min., etc., 1889, i., p. 114. 
^ " Les Mineraux des Roches," par Michel L^vy et Alf. Lacroix. Paris, 1888. 
Baadry et Cie. 218 fig., i pL, pp. 
