Pirsso?i and Wells — Occurrence of Leadhillite. 221 



that given by Laspeyres and approaches the angle measured by 

 Artini* on the Sardinian mineral, ang. /3=89° 31' 55". The 

 plane //, -J-l, (118) first mentioned by Artini, is easily identi- 

 fied in the present occurrence by its lying in the zone 410 /\ 001 

 and by the measurement on the base. It may be mentioned 

 here that all the faces shown in fig. 2 have been greatly exag- 

 gerated except the pinacoids and prisms, in order to exhibit 

 them better. 



One of the most marked peculiarities of leadhillite is its 

 close approach to hexagonal symmetry in certain of its forms. 

 Thus we have, 



fflXm 100^110 = 60° 13' 30" 

 o A w 001/^201 = 68 18 

 c y^e 001^201 = 68 39 

 c A a; 001/^111 = 68 31 

 c a?" 001 A iH = 68 42 



A combination of these planes evidently could not be dis- 

 tinguished by the eye alone from a hexagonal prism terminated 

 by a hexagonal pyramid. The crystals from Missouri often 

 appear strongly rhombohedral from the fact that they have the 

 habit shown in fig. 1, and are repeatedly twinned on the unit 

 prism. Where the edge between the base and a face in the 

 prism zone is replaced by a single plane as in fig. 1, and the 

 reflections are poor it is often impossible to locate this plane 

 and orient the crystals. 



The examination of a cleavage plate in convergent polarized 

 light gives the trace of the axial plane, however, and enables 

 one to decide at once between the pyramids and hemidomes 

 given in the above table, but since the inclination of the angle 

 ft is so slight it is still impossible to tell whether they are in 

 the obtuse or acute angle. In the present case the orientation 

 adopted is that given by the agreement between the best meas- 

 urements and the calculated angles and as these are very close 

 it is regarded as undoubtedly correct. 



Twinning. — As previously mentioned twinning on the unit 

 prism is very common. Cleavage plates in polarized light are 

 also seen to contain lamellae twinned in this way. 



Cleavages by reflected light rarely present a perfectly plane 

 surface, but are covered with slight re-entrant and salient 

 angles in all directions, which reflect, on the goniometer, 

 numerous scattered images of the signal. 



If the mineral was orthorhombic it is evident that the bases 

 of the intergrown twinned, individuals would all lie in one 

 plane, but as it is really monoclinic these small, apparently 

 vicinal angles are due to the slight inclination of a (100) /\ c (001). 



*Giorn. Min., i, 1, 1890. 



