274 Description of FERGUSONITE, 



The substance of Allanite is brittle ; the hardness = 6.0 ; 

 some varieties a little higher, others a little lower, but the dif- 

 ference is scarcely perceptible. The specific gravity is different- 

 ly stated ; but the highest obtained by Count BOURNON, which 

 seems to be that of the pure mineral, is = 4.001. It does not 

 act upon the magnetic needle. 



This description does not differ in any material point from 

 that which has been given by Mr ALLAN, except in respect to 

 the regular forms, which were obtained from specimens disco- 

 vered after the publication of his paper. 



II. FERGUSONITE. 



The regular form of this mineral is one of the most interesting 

 of those comprised within the pyramidal system. The fundamen- 

 tal form, as deduced from approximate measurements, is an isosce- 

 les four-sided pyramid, having its terminal edges = 100 28', and 

 its lateral edges = 128 27', Fig. 7. The character of its com- 

 binations is hemi-pyramidal, like that of Tungstate of Lime, the 

 pyramidal Scheelium-baryte of MOHS, with which it agrees also 

 very nearly in its angles. The character of its combinations is 

 evident from Fig. 3., where the fundamental pyramid is in com- 

 bination with an acute four-sided pyramid, with a rectangular 

 four-sided prism, and with the pyramid of infinitely small axis, 

 or P oo . The angle a b c in Fig. 6., which is the horizontal 

 projection of Fig. 3., being about 11, it follows that the prism r 

 itself consists of the alternating faces of that eight-sided prism, 

 whose transverse section is 112 37' 12", and 157 22' 48", and 

 whose crystallographic sign, according to the method of Profes- 

 sor MOHS, is g 00 ^ The faces of the pyramid are general- 

 ly curved, and yield on that account various measures with the 

 common goniometer, giving the inclination at its base from 158 



