Mr. Levy on Murchisonite. 449 



ages in three different directions, two of which are at right 

 angles to each other, hke the two principal cleavages of com- 

 mon felspar, are obtained with the same facility, and present 

 the same characters. The third has a nacreous appearance, 

 is obtained as easily as the other two, and is found by the re- 

 flective goniometer to be pei-pendicular to one of them, and 

 to make with the other an angle of 106° 50'. So that the 

 solid obtained by cleavage is a letrahedral prism, such as 

 fig. J , the incidences of the planes of which are as follow : 



P,^> = 90°. P, h' = 106° 50'. g\ h' = 90°. 



This substance in the specimens from Dawlish, is white 

 with a slight tinge of red, and is opake; it is accompanied by 

 quartz, a little mica, and very minute crystals of black tour- 

 maline disseminated throughout the mass. The whole forms 

 rather a compact rock ; but in some specimens the substance is 

 partly or entirely disintegrated, almost pulverulent, and of a 

 pure white colour. 



Mr. Brayley jun. having kindly given me for examination se- 

 veral specimens he had himself collected, of the conglomerate 

 of Heavitree, near Exeter, I found disseminated among the 

 minerals and rocks which compose it, a great many crystals of 

 this substance, always rounded on the edges, either slightly 

 adhering to the red marl, or strongly attached to the more 

 solid parts of the conglomerate. The form of these crystals 

 is generally that represented by fig. 1, parallel to the planes 

 of which they readily cleave. 



Another form offered by these crystals is that represented 

 by fig. 2 ; the plane a is always narrow, dull, irregular and 

 curved, and its incidence upon P, measured by thexommon 

 goniometer, is about 120°. Most frequently, however, these 

 crystals are macled. Suppose two crystals of the form fig. 2. 

 first placed in a parallel position, and in contact by their planes 

 g' or penetrating each other; if then one of the crystals be sup- 

 posed to turn round an axis perpendicular to the plane g^ till 

 its face P makes an angle of 128° 10' with the face P of the 

 other, they will be in the relative position of the two indivi- 

 duals which form the macles of this substance. The two na- 

 creous planes are then inclined to each other at an angle of 

 161° 10', and the plane a of one crystal is neai'ly on the same 

 level as the plane P of the other : so that as these crystals are 

 always so much rounded on the edges, and their planes so ir- 

 regular, it is, in the greater number of cases, only by cleavage 

 that it can be discovered that they are macles. 



The nacreous cleavage of these crystals is not alwaj's so 

 easily obtained, and frequently more interrupted than in the 



Ncxv Series. \'ol. 1. No. 6. .func 1827. M M speci- 



