of Crystallized Bodies. ' 61 



The same law applied to the pyramid P -f- 1 (*), on the ter- 

 minal edges of which the faces of P appear with parallel 

 edges of combination, will produce a compound form similar 

 to Fig. 2., the edge x being parallel to the edge x'. The 

 crystallographic signs of these two compositions are P, {f-} 

 and P -f- 1, {£}. The simple pyramid P -f 1, composed 

 in this manner, is sometimes met with in the tin-mines of 

 Schlaggenwald in Bohemia, but it is rare, and much more fre- 

 quently additional faces, particularly those of the two regular 

 four-sided prisms P -f oo and [P -f od] produce forms like 

 Fig. 3. and Fig. 4., in the former of which, the additional 

 faces are yet small, which, in the latter, take the greater share 

 in limiting the figure. The variety of pyramidal Tin-ore, 

 from Goshen in Massachusetts, occurs in the form, Fig. 3. ; 

 the specimen upon which it has been observed, in the cavities 

 of albite, and accompanied by red and pale green tourmaline, 

 is in the cabinet of Mr Allan. The appearance of Fig. 4. is 

 less common in the varieties of the present species, than that of 

 the preceding composition ; it is found in Cornwall. 



It is rare to meet with regularly composed varieties of py- 

 ramidal Tin-ore, in which the groups consist of only two indi- 

 viduals. Much more generally, the composition is repeated 

 either parallel to one plane only, or in the direction of several 

 homologous planes of the pyramid P. In the first case, the 

 particles beyond two faces of composition, resume an exactly 

 parallel position, as in Fig. 5., and if farther repeated, the 

 whole may consist of alternating laminae of two individuals. 

 Another kind of geniculated crystals is produced if the com- 

 position takes place parallel to two faces from the upper and 

 the lower apex, which, if duly enlarged, would meet in a 

 horizontal edge. The variety, Fig. 7., depends upon exactly 

 the same kind of composition relative to the same number of 

 individuals, with this difference only, that they are here con- 

 tinued beyond the faces of composition, so that, if Fio-. 6*. is 

 represented in the crystallographic method of Professor Mohs, 

 by P + 1. T + oc. |P + od], Vi }, the sign of Fig. 7. will 

 be P + oo. [P + x]. CP + ap) s ,2{£} : Very often every 

 trace of the faces of four-sided or eight-sided pyramids, 



