Description of a New Mineral called Haytorite. 299 



considered a new mineral, its primary form (assuming the 

 planes P and k, k' as primary. Plate IV. Fig. 9.) is an oblique 

 rhombic prism, differing less than 1° from the proportions of 

 a right rhombic prism, and of which the lateral planes meet 

 at the angles of 77° and 103°, the terminal plane declining 

 from one acute angle to the other. Some of the crystals 

 forming one large groupe in my possession are opaque; in 

 others translucency exists ; and others again seem to have 

 suffered a partial decomposition, having the appearance of 

 being carious internally ; but to what extent soever that ap- 

 pearance has taken place, it is remarkable, that the portion 

 remaining of the external plane, however small, is not de- 

 prived of its ordinary lustre, and often is even brilliant. 



Now, if this apparent injury had been the real effect of de- 

 composition, we might expect that the agent producing it 

 would, in the first place, have acted externally, and thus have 

 deprived the external planes of their natural brilliancy ; and 

 this consideration again tempted the farther examination, as 

 to whether there existed in the crystals thus partially hollow- 

 ed, any stalactitic appearance of chalcedonic matter. This 

 certainly does appear, on a close examination by the help of 

 a glass, a circumstance amounting almost to proof that the 

 crystals are in reality pseudomorphous, and that their sub- 

 stance is chalcedony. The smaller crystals were sometimes 

 enveloped by it. 1 ' 



Mr Phillips has given the following measurements in refer- 

 ence to Fig. 9. of Plate IV. 



