1894.] Mineralogy and Petrography. 169 
‘thin section. As large as is this number it is exceeded in sections of 
the basic concretions of the rock from Niedersteina. These concre- 
tions according to Hermann’ are made up largely of hornblende and 
cordierite, and thousands of apatites, sometimes reaching 1200 ina 
single section. The interesting features of these apatites is not, how- 
ever, their number, but their forms. In many cases they are skeleton 
crystals whose many branches are parallel like the teeth of a comb. 
The Hour-Glass Form of Augite.—This well known form of 
augite, according to Blumrich,” is usually connected with zonal growth 
in the mineral, and is limited in its occurrence to the pyroxene of alka- 
line rich magmas. It is found not only in augite, but also in other. 
minerals forming colored isomorphorus mixtures. The hour-glass form 
owes its existence to the fact that different crystallographic faces in a 
growing mineral attract molecules of different chemical compositions, 
which by addition to the attracting faces build out these faces with 
differently coloredsubstance. The structure is certainly not due to the 
filling in of the outlines of skeleton crystals, as has often been as- 
sumed. Zonal bands extend uninterruptedly through both dark and 
light areasin the crystals, hence the materials of both must be of the 
same age. The one cannot have been a later deposition than the 
other. Pelikan" in confirmation of Blumrich’s view, calls attention to 
the fact that if strontium nitrate crystals be allowed to grow in cer- 
tain colored solutions, they become colored in areas distributed in 
accordance with the faces by which the crystals are bounded. The 
central cores of chiastolite crystals, Becke ascribes in a similar man- 
ner to the attractive influence of the end faces of the crystals upon the 
material added during growth. 
The Effect of Impurities in Crystallizing Solutions.—It 
has long been known that the habit of crystallization assumed by a 
substance depends in large measure on the medium from which it ery- 
tallizes. Araganite, for instance, will separate from certain solu- 
tions, while from others calcite is precipitated. Vater” has conducted 
a series of experiments with calcium carbonate, allowing this substance 
to crystallize from various solutions under different conditions; and 
has reached some interesting conclusions. The ground rhombohedron 
*Neues Jahrb. F. Min., ete., 1898, II, p- 52. 
Minn. u. Petrog. Mitth., XIII, p. 239. 
"Tb. XIII, p. 258. 
“Zeits. f. Kryst. XXI, p. 433 and XXII, p. 209. 
