1835.] Mineralogy and Petrography. 501 
dium light. The real optical angle, 2V,, is 55° 20’. These 
results agree very closely with those obtained by Hjortdahl. 
ans Thürach' has contributed an interesting paper on the wide 
distribution of zircon and certain titanium minerals as microscopic 
rock-constituents. Decomposed rocks were especially investi- 
gated, from which these minerals were the more easily separated, 
Rutile, anatase, brookite and pseudo-brookite were all identified, 
Tourmaline, staurolite, garnet and some other minerals are also 
spoken of in the same connection, and a long list of localities 
given where all these substances were observed. Kal s 
finds that in certain rocks, especially nepheline-basalts from Ran- 
den in the Hegau, Baden, and from Tharand in Saxony, twins of 
olivine are quite common. The twinning plane is a brachydome 
whose angle over OP is nearly 60°, as was observed by vom Rath 
in free crystals of monticellite from Mte. Somma. Knop’ has 
made a thorough chemical study of the augite occurring in the 
various rocks of the Kaiserstuhl in Baden. One group is inter- 
esting on account of their'containing TiO,, the amount sometimes 
exceeding four per cent. This would naturally be supposed to 
isomorphously replace SiO., but on account of the violet color of 
the augite in which titanium is most abundant, it is suggested 
that this element may also be present as Ti,O;, replacing ferric 
iron. Schuster* adds over 200 pages to his former paper on 
the crystallography and structure of danburite, making his numer- 
ous and careful measurements the basis of general conclusions 
regarding the nature of forms possessing very large indices, to 
which Websky has applied the name “ vicinal-planes.” For the 
many important results obtained, reference must be made to the 
original article. Vicinal-planes are found (p. 490) to possess a defi- 
nite relation to some principal plane having simple indices, wi 
which they are associated. This relation is a genetic one. Vicin- 
al-planes are regarded, so to speak, as “induced” by the joint 
action of two forces, one exercised by the new molecules in their 
effort to form a really new plane and the other exerted by the old 
plane to retain its exact position. The crystalline form of - 
the element thorium has been for the first time determined 
by Brogger. Although apparently rhombohedral, the minute 
crystals (only 0.15™™ wide and 0.015™™ thick) are really 
ular, being a. combination of a cube and octahedron. 
Schaeffer? describes a new American locality for tantalite, the Etta 
tin mine in Dakota. Its composition is TaO, = 79.01; SnO, = 
1 Ueber das Vorkommen mikroskopischer Zirkone und Titan-mineralien, Würz- 
burg, 1 
2 Poitschaih fiir Krystallographie, Vol. x, p. 17, 1885. 
3 Zeitschrift für Krystallographie, Vol. x, p. 58, 1885. 
4 Tschermak’s Min. und Petr. Mittheilungen vi, pp. 301-515, 1885. 
5 Meddelanden fran Stockholms högskola, No. 1, 1883. 
6 American Journal of Science, Dec., 1884, p. 430. 
