192 W. G. Brown — Quartz-tioin from Virginia. 



composition face R. Indeed he supposes that it was formed 

 by the growth of quartz crystals upon the — iH faces of cal- 

 cite in a certain definite manner which he explains. 



A little later Jenzsch* in a paper on the twinning laws of 

 quartz, under the second law, which he calls the " Reichen- 

 steiner" or Gr. Rose's, seems to admit the explanation of Eck as 

 applied to the whole group described by Rose, but says that 

 for each two crystals in twinned positions Gr. Rose's law holds 

 good, and gives an ideal figure representing two crystals twin- 

 ned according to the law, twinning plane R. This figure (197 

 B) is in Dana.f Jenzsch seems to consider the statement of 

 NaumannJ that the twinning plane in these crystals instead of 

 R may be — fR " R (oder auch — f-R)" as a quibble or attempt 

 at a complete bibliography which it was hardly worth noticing. 



That the view of Eck was the correct one is strengthened by 

 the paper of vom Rath and Frenzel§ and that of E. S. Dana,| 

 in both of which regular growths of quartz on calcite are 

 described which, while not exactly agreeing with the groups 

 described by Rose, seem to leave no doubt but that groups of 

 quartz, " pseudo-twins," are formed by regular growth upon the 

 rhombohedron — -JR of calcite. In the paper of vom Rath 

 and Frenzel it is said that the faint reddish color of the Reich- 

 enstein quartz which reminds one of pale amethyst is due to 

 disseminated particles of hematite. They further redescribe 

 the group of G. Rose and give an exact drawing of the crys- 

 tals (naturtreue Zeichnung). 



Hare^f says the form described by Rose is not excessively 

 rare at Reichenstein. and mentions, as do both vom Rath and 

 Frenzel and E. S. Dana, that Breithaupt first noticed the regu- 

 lar growth- of quartz on calcite and gives full references to 

 Breithaupt's papers. 



Since Jenzsch** says " Obgleich nun das Gr. Rose'sche Gresetz 

 mit Evidenz bis jetzt nur von Reichenstein in Schlesien be- 

 kannt 1st," and in the last edition (11th) of Naumann-Zirkel 

 the view of Eck with regard to this kind of twinning is accep- 

 ted, it appears justifiable to describe a group of amethystine 

 quartz crystals found near Mediums River, Albemarle County, 

 Virginia, in which there is twinning according to the law. 

 Twinning plane and composition face — §R. 



The group, which is transparent and of decidedly amethys- 

 tine color is represented in nearly actual size by the accom- 

 panying figure. The sub-individuals marked a and c in par- 



* Poggen. Armalen, vol. cxxx, p. 600, 1867. f Loc. cifc. 



iElernente der Mineralogie, 6. Ed., p. 190, 1864. 

 § Poggen. Annalen, vol. xvii, p. 155. 1875. 



|| American Jour, of Science, III, vol. xii, 448, 1876, and Zeitschr. fur Krystall., 

 vol. i, p. 39, 1877. 



If Zeitschr. fur Krystall., vol. iv, p. 244, 1880. ** Loc. cit. 



