52 M. Rose on Felspar, [JAN. 



edge G, into two angles of 60° 8' and 57° 45", the first of which 

 has one of its sides situated in the plane M, and the second, one 

 of its sides in the plane T. The section perpendicular to the 

 planes M and P is an oblique-angle parallelogram, the obtuse 

 angle of which is divided by the plane n, produced by a decre- 

 ment by one row on the edge B, iuto two angles of 46° 5' and 

 47° 31'; the first of which corresponds to the edge of the paral- 

 lelogram situated in the plane P ; the other to the edge of the 

 parallelogram situated in the plane M. 

 The planes I have observed are 



PMT G* G 4 a HAAB C C (see figs. 3, 4). 



I x f y x n o g 



Incidences.* 



TonM' 117° 53' 



Ton/ 122 15* 



Moa/ 119 52* 



Monz 149 12 



lonz 150 40 



M'on/ 148 30 



T on/ 149 23 



PonT 115 5* 



Pon/ 110 51* 



Pono' 122 23* 



Mono' 112 11 



P on g 150 5 



Mong 100 52 



PonM 86 24 



Pon« 133 55 



P on y 97 37 



T on y 134 32 



Ton*' 110 29 



Pons' 127 23 



Plane Angles of the Primitive Form. 



Those of plane P 119° 12' and 60° 8' 

 M 116 35 63 25 



T 99 45 80 15 



The crystals of albite are frequently or almost always met 

 under the form of hemi tropes. t These hemitropes are formed 



• I have marked* the angles from which the others are calculated. 



+ I found, however, afterwards, that the crystals of St. Gothard, the prisms of 

 which are so short that the planes of one of the summits meet those of the Other, are 

 very likely albite : they are met commonly in simple crystals, and seldom in hemi- 

 tropes. Their planes were not sufficiently brilliant to be measured ; but it is likely that 

 they were albite, since, when digested in hydrochloric acid, they were not decomposed. 



