OPAL PSEDDOMORPHS FROM WHITE CLIFFS — ANDERSON AND JEVONS. 33 



uypsum. Later Weisbach^ measured the angles and came to tlic 

 conclusion that the original mineral was oi'thorhombic in system ; 

 lie suggested sulphur. He was followed by Pelikan'', who 

 compared them to aggregates of gypsum crystals. Giirich" gives a 

 more detailed account and concludes that the original mineral 

 was monoclinic and probably identical with the original of the 

 well-known " barley-corn"' pseudomorphs from Sangerhausen, and 

 similar pseudomorphs from elsewhere. But even if this conclu- 

 sion be justified it does not settle the question, for at least five 

 mifierals have been suggested as the original of the 8angerhausen 

 and similar specimens, celestite, perhaps, being regarded as the 

 most likely,^ 



Description nf SpecimPMs. — The material for this paper was 

 furnished by two specimens in the collection of the Geological 

 Department, Sydney University, and five from the Australian 

 Museum collection. That represented in PL vi., fig. 1, is the 

 largest and best developed, hence it has supplied the bulk of the 

 angular measurements by the contact goniometer. Unfortunately 

 it is found that the angles vary somewhat, thus giving an element 

 of uncertainty to the conclusions di"awn therefrom ; yet, by 

 making a large number of measurements and taking means, it is 

 hoped that a fair approximati(m has been made to the true 

 angles. 



The seven specimens vary in their greatest diameter from 1 1 

 cm. to 7"5 cm. approximately. They present a fairly unifoini 

 appearance, which is that of an iii-egular, radial aggregate of 

 acute, tapering, four-sided pyramids. Owing to the curvature of 

 the faces it is scarcely possible to secure exact measurements of 

 the angles, tliough an attempt was made to counteract this source 

 of error by making the goniometer arms tangent tt) the part of the 

 faces close to the edges. An important feature in most of the 

 pseudomorphic crystals is the w(>ll-mark(!d cleavage (PI. vi., fig. 2)" 

 Tt generally crosses one <mly of the four terminal edges, but 

 sometimes passes over the apex and appears, though less strongly 



6 Weishach— Neues Jahrb., ii., l.syS, p. 150. 



» Pelikan — Tsehermak's Min. petr. Mitth.,xix., 1900, p. 336. 



« Giirich— Neues Jahrb., Beil. Bd., xir., 1901, pp. 478-483, fit;. 



9 Dana— Bull. U.S. Geol. SurT., 12, 1884, pp. 2.5 28 ; Sjst. Minora) . 

 Gth Ed., 1892, pp. 271, 907. 



Miers— Min. Map^., xi., 1897, p. 26*. 



