Want of Identity between Microlite and Pyrochlore. 117 



I shall first exhibit the mineralogical differences between the 

 two minerals, in order to show the fallacy of any attempt to unite 

 the two species on natural-history grounds ; and shall then inquire 

 what support the new view acquires from chemistry. 



Pyrochlore. Microlite. 



Crystals, unmodified regular Crystals, regular octahedroHj 

 octahedrons. with edges truncated and angles 



surmounted by four-sided pyra- 

 mids, whose faces correspond to 

 the octahedral planes. 



Cleavage, octahedral, distinct. 



Cleavage, none (fracture con- 

 choidal). 



Lustre, resinous to vitreous. 

 Color, deep red-brown to black. 



Streak, brown. 



Translucent on the edges, to 

 opake. 



Hardness = 4.75. 



Gravity=4.20to4.25. 



Lustre, resinous. 



Color, pale honey-yellow. Sur- 

 faces of crystals often reddish or 

 blackish brown from implanted 

 minerals. 



Streak, pale yellowish white. 



Semi-transparent to translu- 

 cent. 



Hardness, 5.25 (readily 

 scratches Pyrochlore). 



Gravity =5.485 to 5.562.* 



I consider it apparent therefore, that all attempts to unite two 

 minerals, whose individuals afford no transition-links to constitute 

 the passage of insensible gradation, in properties so essential as 

 specific gravity, internal structure and hardness, must prove una- 



* The specific gravity as heretofore quoted, was 4.75 to 5.00 ; but very oppor- 

 tunely for this examination, I had brought with me from my Charleston cabinet 

 an excellent crystal, weighing 3.805 grains ; and possessing one of Robinson's best 

 eight inch-beam balances (which turns readily with one thousandth of a grain), I 

 was able to correct the specific gravity, as above. The original determination of this 

 properly was eflTected by means of a balance much less sensitive, and on a crystal 

 whose weight was only four tenths of a grain. Indeed, this was tfie lamest 

 crystal I had seen, when I described the species. Observing however, tiiat my 

 crystal of 3.805 grains had two of its octahedral sides pitted from tJie implanta- 

 tion of calcareous spar and green tourmaline, I at first took its specific gravity with 

 these impurities attached. The result was 5.485. I then cleaved off' these faces 

 thereby reducing the weight of the crystal to 2.410 grains, when its gravity rose 

 to 5.562. 



