Mr. Levy on Murckisonite. 4' •51 



and in Bohemia, and represented in the translation of Mohs's 

 " Mineralo^v," by figures 80 and 8 1 . For in both, the axis ot 

 revolution i^' perpendicular to one of the two cleavages at 

 riaht anales to each other, and in felspar the second cleavage 

 of one of the individual crystals forming the made is mchned 

 upon the second cleavage of the other, at an angle ot 12/ o, 

 according to the dimensions I have assigned to the prmntive 

 form of felspar; whilst in the crystals under consideration, the 

 same angle is about 128°. , 



The onlv difference which is now left to distinguish this 

 mineral from felspar, whether in its laminar form, as in the 

 specimen from Dawlish, or in crystals, as ni those trom 

 Heavitree, is therefore the nacreous cleavage which it pos- 

 sesses, under both forms, and which cannot be obtained in 

 felspar. But not only this cleavage does not exist in the va- 

 rieties of felspar which have hitherto been exammed, but it is 

 not parallel either to any known modification of that sub- 

 stance, or to any unobserved modification, which might be 

 derived by some simple law from the primitive torm. lo 

 show the truth of what 1 have advanced, it is sufiicient to ob- 

 serve, that in order to compare fig. 1. with a crystal of e- 

 spar,-for instance, with the figure given m Mr. W.Uiil- 

 lips's "Mineralogy," we must suppose that the plane F, tig. J, 

 corresponds to his plane P, and the plane g\ to his plane M. 

 Then we ought to find that the plane h' corresponds to 

 either of his planes c\ c\ c\ which are perpendicular to M, 

 and inclined to P. But this is not the case : ^r these planes 

 are respectively inclined upon P, at angles ot 99 la , 1^9 ^9 . 

 and 146° 3', whilst the inclination of the plane h owV, tig. l, 

 is 106° 50'. Moreover, I find that a plane, the inclination ot 

 which upon P would be equal, or nearly equal, to 106 50, 

 could only be derived by one of two laws, trom the oblique 

 rhombic prism, which is the primitive form of felspar, either 

 by the law o-fox ai. Now neither of these modifica- 

 tions has ever been observed in felspar, and they are rather 

 beyond the simplicity which might be reasonably expected in 

 a modification parallel to which a cleavage is lound to exist 

 in some varieties. If now it is remembered that, as tar as 

 crystallographic observations go, it is tound that, althougn 

 .ome varTeties of a species present occasionally cleavages which 

 do not exist in all, in no occasion a cleavage has been ob- 

 tained which did not correspond to some simple modihcation ; 

 even the fUlse cleavage or faces of composition,-thcn the ground 



upon which I would propose to consider the substance I have 

 iust now described as distinct from felspar, will I hope be- 

 'come sufficiently obvious. The definition of the mmeralo- 



