290 C. ANDERSON. 



the crystallographic features and the complicated twinniug 

 of the three occurrences. The cerussite was found on the 

 roof, sides, and floor of vughs in the ore masses, associated 

 with manganic iron oxide, anglesite, smithsonite (zinc 

 carbonate), embolite, and galena; in some specimens small 

 crystals of galena are seated on the cerussite, indicating 

 that the former results in these cases from secondary 

 deposition. In habit the crystals vary somewhat. Simple 

 crystals are rare, twins being the rule with r (130), or m 

 (110) as twin plane. When doublets occur they are, so far 

 as I know, always twinned on the r law and the crystals are 

 either of arrow-head shape (Plate LIV, fig. I), 1 or pris- 

 matic along the vertical axis. 2 A group of arrow-head 

 twins scattered on a matrix of oxide of iron (Plate LIV, 

 fig. 4) forms a specimen of rare beauty; it will be observed 

 that the crystals are generally attached to the matrix by 

 the point of the 'arrow.' The reticulated or dendritic 

 groups, which are the commonest of all, and which may be 

 described as tabular on b and elongated parallel to the a 

 axis (Plate LIV, fig. 2) are combinations of twins on r 

 and on m, and may consist of a dozen or more individuals, 

 forming a polyet 3 of a complicated nature. The present 

 paper is mainly concerned with the elucidation of the 

 interesting features presented by these polyets. 



Measurements were made on a Goldschmidt two-circle 

 goniometer, which is well adapted for the investigation of 

 complicated groups such as are here described. Measure- 

 ment is greatly facilitated by the fact that the vertical 

 axes of the several individuals (segments) composing the 

 polyet are parallel, so that all necessary angular determin- 

 ations can be readily made with one mounting of the group 



1 Anderson, loc. cit., pi. lxxvii, figs. 1, 2. 8 Id., ibid., pi. lxxvi, fig-. 3. 



3 I am not certain whether this term has previously been used for a 

 group of several twinned crystals, but an English equivalent for the 

 German 'vielling' is necessary, and polyet seems a suitable word. 



