272 Description of FERGUSONITE, 



species occurs along with the Allanite, and sometimes imbed- 

 ded in it, in small but very distinct crystals, generally of the 

 form of a four-sided prism, terminated by a four-sided pyramid, 

 in parallel position, and having occasionally its lateral edges re- 

 placed. HAUY, and most of the other mineralogists, in the de- 

 scription of Allanite, have quoted the measurement of the prism 

 117, as indicated by Dr THOMSON. Messrs PHILLIPS* and 

 BROOKE f ascribe to it a rectangular four-sided prism, which 

 is incompatible with the real form of the mineral. That sub- 

 stance, the form of which Mr PHILLIPS has described, and 

 which he calls Crystallised Allanite, does not belong to this spe- 

 cies : it is a species of its own, but, like the AUanite, it belongs 

 to the order Ore, of the system of Professor MOHS. 



The more particular object of the present paper, is to de- 

 scribe this new species, for which, at the suggestion of Mr ALLAN, 

 I propose the name of Fergusonite, in honour of a gentleman 

 too well known to the mineralogical world at large, and to the 

 members of this Society in particular, to require, in the present 

 place, a more detailed acknowledgment. In order to produce a 

 more distinct idea of the differences existing between the two 

 species, I shall begin with giving a short description of the Al- 

 lanite, and then add the description of Fergusonite, as derived 

 from the specimens observed in the cabinet of Mr ALLAN. 



I. ALLANITE. 



Its form is tetarto-prismatic. Plate XII. Fig. 1. shews the 

 elevation of a crystal. Fig. 2. the projection of the same, upon 

 a plane perpendicular to the sides of the prism. It is the same 

 crystal which has been mentioned by Dr THOMSON, and presents 



* Ekmentary Introduction to Mineralogy, p. 264. 

 J- Familiar Introduction to Crystallography, p. 458. 



