154 



The first is a pearl spar of the usual light appearance, the 

 second more coloured with iron, the next more so still, 

 and the last most of all. They may perhaps contain a little 

 manganese. The outline shows the position of the primi- 

 tive rhomb in the upper figures, in the others it is situated as 

 in the asquiaxe. Some specimens of these were sent me by 

 Lord Heathfield from Devonshire. I have had others of 

 nearly the same nature from the Isle of Man by favour of the 

 Duke of Athol and Lord Henry Murray. They seem to 

 indicate iron in their neighbourhood, and may be useful in 

 smelting it; but are themselves very deceitful, their ap- 

 pearance giving a stronger indication of iron than be- 

 longs to them ; which will in general be detected most 

 readily by breaking, as the fresh fragment discovers them to 

 be a mere limestone with a slight pearly tinge, which on 

 being exposed to the common air and water will assume the 

 same deceitful tinge as the former exposed parts. These 

 have generally been reckoned among the lenticular ores, and 

 may have deceived many by their external appearance. 



TAB. LXIII. 



This is nearly the same, with very flat lenticular crys- 

 tals standing edgeways, of a dull rusty appearance, the 

 matrix nearly of a similar substance, with some lustre. The 

 whole shows the gradation of tints, and the fresh fractures 

 are lightest, as is common in these varieties. The left hand 

 figure has some signs of the triangular and other faces partly 

 remaining; in the right hand figure they are entirely lost: 

 the middle figure exhibits a transverse section, fresh broken, 

 with sis;ns of the confused rhomboidal fracture. 



