W. G. Brown — Quartz-tivin from Virginia. 191 



pressions from a survey of the earth's features. I was made to 

 see a system of arrangement in the Pacific islands, instead of a 

 "labyrinth;" to appreciate the vast length of the island chains 

 in the great ocean with their many parallelisms, and the accord- 

 ant relations subsisting between them and long lines of atolls. 

 I was thence led to observe the corresponding system in the 

 features of the continental lands, and the more fully so when 

 afterwards it was proved that geology was not in America 

 merely the study of strata and fossils, but of the successive 

 stages in a growing continent. Thus a conception of the earth 

 as a unit became early implanted, and the idea also of its devel- 

 opment as a unit under movements as comprehensive as the 

 system in its feature-lines. My faith in any mountain-making 

 theory hitherto proposed is weak. But that idea of system in 

 structure and progress stands, and, however much ignored by 

 students of the earth's stratigraphy, it must have its explana- 

 tion in a true theory of the earth's dynamics. 



Art. XXY. — On a quartz-twin from Albermarle County, Vir- 

 ginia; by W. G-. Brown. 



In 1851 Gr. Rose* described a group of quartz crystals from 

 Reichenstein in Silesia, and announced that he had observed 

 among them a new twinning law which he states as follows : 



" Die Zwillingsebene ist namlich, eine Hauptrhoniboederflache ; 

 die Krystalle sind aber nicht mit dieser, sondern mit einer darauf 

 senkrechten Flache verbunden, und die Krystallgruppe besteht 

 auch nicht aus zwei, sondern aus vier Individuen, indem an einen 

 mittleren Krystall drei Individuen so angewachsen sind, dass eine 

 Hauptrhoniboederflache von jedeni der letzteren mit einer der 

 drei Hauptrhomboederilachen des mittleren Krystalls in gleicher 

 Ebene liegt." 



In a note to this paper he mentions a specimen from the 

 same place in the possession of Weiss, of which the transparent 

 quartz crystals possessed a slight amethystine color; a color 

 which seems to be common with the other occurrences to be 

 mentioned. J. J). Danaf calls this group of Rose " a three-rayed 

 twin, consisting of a central crystal twinned to three others by 

 each R of one extremity," and gives the figures (figs. 198 A. 

 B.) copied from Rose's paper. 



Bck^; took however a different view of the nature of this 

 group and showed that it was not a twin according to the law 



*Poggen. Annalen, vol. lxxxiii, p. 461, 1851. 



f A system of Mineralogy, 5th Ed., p. 161, 1869. 



jZeitschr. d. deutsch. geol. Gesellschaft, vol. xviii, p. 428, 1866. 



