MINERALOGY. 35 



20, Tetrahedrite [Cug Sba S7]. 



This mineral, elsewhere so important, is very rare in New Hampshire. 

 It has been found in Cornish, associated with stibnite; but the place 

 from which the specimens were obtained is now unknown. 



21. Fluorite [Ca FL]. 



There are several noteworthy occurrences of fluor spar in our state. At 

 the Notch it is found in beautiful sea-green octahedrons, of the size of 

 hickory nuts and of perfect form. It occurs in the quartz veins. In the 

 more exposed portions of these veins octahedral cavities are found, from 

 which the fluor spar has been dissolved, and often these cavities are par- 

 tially refilled with quartz, thus showing the process of the formation of 

 pseudomorphs by replacement ; for, if the process of filling had been 

 complete, we should have octahedrons of quartz just like those that come 

 from Cornwall. These green octahedrons are found on Mts. Crawford 

 and Webster, at Bemis brook, and, indeed, all along the White Mountain 

 Notch. Fluor spar forms a vein of considerable size at Westmoreland, 

 from which crystals weighing several pounds have been obtained. The 

 color is light green, and the crystals are cubic. It is also found at the 

 tin mine in Jackson, where crystals of various colors — green, white, and 

 purple — are found. A pretty purple variety is found associated with 

 albite at Grafton, and also at Newbury. 



Fluor spar, when treated with sulphuric acid, is decomposed with the 

 generation of fluor-hydric acid ; but if a crystal with bright faces is placed 

 in the cold acid for a short time, and then is removed, washed, and exam- 

 ined with the microscope, it will be seen that it is not uniformly eaten by 

 the acid, but that its surface is covered with little depressions bounded 

 by crystallographic faces, which bear a definite relationship to the out- 

 lines of the crystal, and are supposed to indicate certain structural lines 

 according to which the crystals are built. If, now, one of these green 

 octahedral crystals from the Notch is broken so as to obtain a fine 

 bright cleavage surface, and is then submitted to the action of sulphuric 

 acid, it is etched by the cold acid with the greatest ease, much quicker 



