THK NATURAL HISTORY SURVEY. 



45 



quently our samples belong to the variety called almandite. 

 None of the manganese, calcium, or chromium garnets have 

 been found here. 



Tourmaline. ([A1OL, Mg, Fe, Na 2 , Li,, H a ) 3 [Al O'B O] [Si O 4 ] 2 



(Groth) Hexagonal 



The representatives of this species in our region are more 

 or less well crystallized in bedded or standing crystals in which 

 the typical plane of tourmaline appears, as for example the 

 rhombohedral ending to prisms which have three, six, and nine 

 sides (Fig. 18). 



Fig. 18. 



The crystals are usually slender, repeated, forming curved 

 continuations of each other, or radiated groups. The prismatic 

 faces are almost invariably strongly striated verticarly. No dis- 

 tinct cleavage is found in tourmaline, but in slender crystals a 

 parting transverse to the c axis is evident. The fracture is sub- 

 conchoidal to uneven. The tenacity brittle or friable, the 

 hardness 7 to 7.5 and the s. g. 2.98 to 3.2. The lustre is vitre- 

 ous to resinous. The only colors that occur in the specimens 

 of this region are black of a brownish or bluish shade. The 

 streak is uncoloied. They are opaque to transparent in thin 

 sections. Under the microscope tourmaline is strongly dichro- 

 itic, and the absorption parallel to c is greater than that, par- 

 allel to /7. O is dark blue to brown. E is light blue. The 

 polarization colors are brilliant, between a brown and a red. 

 Before the blowpipe tourmaline fuses with difficulty, and is not 

 decomposed by acids. 



One of the readiest and most satisfactory means of dis- 

 tinguishing tourmaline from amphibole and pyroxene is to fuse 

 on a platinum wire a mixture of the mineral with a like 

 amount of acid potassium sulphate and fluorspar, when the 

 boron green flame shows tourmaline. Its superior hardness, 

 absence of cleavage, remarkable absorption when the. axis is 



