10 Mr. Levy on the Identity 



of Heulandite ; since in that form the line joining the angle 

 o with its opposite, makes with the edge h an angle of 89° 5', 

 and also since the faces of the crystals, according to Dr. Rose's 

 statement, are most of them uneven and dull, and do not ad- 

 mit of measurement by the reflective goniometer. If we now 

 compare the measures of the angles of the two substances, we 

 find in Epistilbite M on M (fig. 4) = 135^ 10', and in Heu- 

 landite Z>' on b^ = 135° 52'. The cleavage of the first substance 

 being parallel to a plane, bisecting the angle formed by the 

 planes M, and the cleavage in the second being parallel to a 

 plane bisecting the angle formed by the planes 6'. The faces 

 M (fig. 4) may therefore be considered as corresponding to 

 the planes of the modification Z>' of Heulandite. The difference 

 of 42' between the two angles which we suppose thus to be 

 equal, may very well be accounted for by the imperfection of 

 the crystals of Epistilbite. But I may even add, that having 

 found in Sir Abi-aham Hume's collection a specimen from 

 Faroe, covered with crystals, answering perfectly the descrip- 

 tion given of Epistilbite, I was allowed to detach a few of 

 them, and by cleaving them deeply so as to leave only a very 

 narrow part of the planes M, I could measure the incidence 

 of M on M by means of the reflective goniometer, and found 

 it constantly from 5 to 10 minutes less than 136°. It still re- 

 mains to show the cori'espondence of the other planes s, t, u 

 (fig. 4) with some simple modification of Heulandite. This 

 is the object of the following comparative table; and the near 

 agreement between the angles of the planes of Epistilbite, and 

 the calculated angles of the modifications of Heulandite, prove 

 I believe satisfactorily, that the planes s, (fig. 4) may be con- 

 sidered as the result of a decrement by two rows along the 

 lateral edges h of the primitive of Heulandite ; two of the faces 

 t as the result of a decrement by one row on the angle o ; and 

 the two others of a decrement by two rows in height on the 

 angle a ; two of the faces tc as the result of a decrement by two 

 rows in height on the edges d ; and the two others as the re- 

 sult of a decrement by three rows in breadth on the lateral 

 angles a. 



Calculated angles of the modifications Angles of Epistilbite 



of Heulandite. according to Dr. Rose. 



h^, h" = 147° 40' 40" s, s = 147° 40' 



6', 6' = 135° 52' M,M=135°10' 



o\ i' = 122° 15'-* 



ai,6;=123°19'} /,M=122°9' 



oSa^ = 108° 29' ...... i, ; = 109°46' 



6?%o'= 154° 31/-I 



«3,«*-154°ll'| M.= 154°51' 



Fig. 



