664 SCIENTIFIC RECORD FOR 1883. 



of the three known species of plagioclase, containing respectively cal- 

 cium, sodium, and potassium. He shows, also, that there is the same 

 sort of correspondence in optical characters as that proved by Schuster 

 for the varieties of plagioclase containing no potassium. 



A number of other important memoirs on the feldspars can only bo 

 alluded to here, as those of Wiik on the feldspars of St. Gotthard and 

 of Filmland (Zeitsch. Kryst., vn, 76; viu, 203), and of Bentell (Zeitsch. 

 Kryst., viu, 351^, on some Silesian feldspars of the potassium -sodium 

 type. More important than these is a new memoir by Des Cloizeaux 

 (Bull. Soc. Min. France, vi, 89), who has contributed more to our knowl- 

 edge of the optical characters of the members of the group than any one 

 else. He has made a new determination of the angle of the optic axes, 

 the orientation of their plane, and of their bisectrices, and of the dif- 

 ferent kinds of dispersion in a large number of varieties of albite and 

 oligoclase. As the result of these, he admits that the relations shown 

 by Schuster exist in many cases, but he finds numerous exceptions, 

 and concludes that these relations are not generally true as claimed by 

 Schuster. The same author has also described a soda-orthoclase from 

 the island of Quatre Eibeiras (Ibid., vi, 107), which is interesting be- 

 cause it corresponds so closely to the varieties found by Forstner in 

 Pantelleria as described above. 



The discussion in regard to the cause of the so-called optical anoma- 

 lies observed in many crystals — alluded to at some length in the last 

 volume of this Report — has been actively carried on during the past 

 year. The most important contribution to the subject is that of Buck- 

 ing (Zeitsch. Kryst, vn, 555), who has investigated the effect of a known 

 pressure upon doubly refracting minerals, following out a line of inves- 

 tigation already entered into by himself, Klocke, and others. Experi- 

 ments were made with sections cut transverse to the optic axis of apatite, 

 beryl, and tourmaline, and one of sanidine transverse to the acute bisec- 

 trix. With apatite the pressure changed the uniaxial figure, seen in 

 converging polarized light, into a biaxial figure, the axial angle appear- 

 ing in the plane perpendicular to the pressure, lu the case of beryl, the 

 section employed showed already a biaxial figure with the angle in the 

 direction of the pressure; pressure diminished this, and as it was in- 

 creased an axial angle perpendicular to the direction of pressure arose. 

 Besults similar to the above were obtained with tourmaline, though the 

 axial angle was smaller than in the other cases, and even a pressure of 

 100 pounds produced no change that was permanent. It need hardly 

 be added that experiments such as the above are most conclusive in 

 showing that pressure, as produced by internal tension, must in many 

 cases be the true explanation of the optical anomalies. 



Brauns (Jahrb. Min., 1883, n. 102) has made another valuable contri- 

 bution to the-snbject. He has followed Klocke in investigating alum, 

 lead nitrate, and some other salts. His conclusions are that chemically 

 pure crystals (of alum, &c.,) are completely isotropic, while the anoma- 



