348 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



No occurrence of topaz in eruptive rocks has been previously described, so far 

 as is known to the writer. Topaz is found in other parts of the Kocky Mountains, 

 and iu Mexico, where eruptive rocks are said to occur, but the connection between 

 the two has not been demonstrated. 



The satin-like luster of the sanidines. The lustrous surface is in the orthodiagonal 

 zone and inclined a few degrees to the orthopinaeoid, as is evident iu the Carlsbad 

 twins, usually polysyuthetic, the luster reaching its maximum of brightness simul- 

 taneously iu alternate planes. Microscopical investigation shows a most perfect parting 

 parallel to the surface of luster, and with a knife-blade flakes can be split oft' in this 

 direction, even more readily than parallel to the basal cleavage-plane. Thin plates 

 parallel to the base (OP) show a very fine striation at right angles to the Hue of cc P So 

 and to the directions of extinction. Thin flakes split ott' parallel to the lustrous 

 surface show, under the inicioscope, that the luster is due to interference of light iu 

 passing the films of air between the extremely thin plates produced by the parting. 

 The thinnest flakes, composed of a few plates, are transparent and exhibit delicate 

 colors of interference, while those composed of more plates arc dull translucent or 

 opaque, the light having been completely extinguished by the repeated interference. 

 The luster is then due to reflected light from the air tiluis near the surface and to its 

 interference. By examination with a good hand leus, a delicate play of colors may 

 be seen upon the lustrous surface of the crystals. 



In the drusy cavities above described the sanidiues are thin tablets, almost invari- 

 ably Carlsbad twins, with prominent development of the clinopinacoid. Such crystals 

 examined under the microscope, as they lie upon the predominant 

 pinacoidal face, aflbrd a means of determining approximately the 

 position of the plane parallel to which the parting referred to takes 

 place. The adjoining cut represents one of these crystals, a nor- 

 mal Carlsbad twin, with a third and smaller plate, also in twin posi- 

 tion. The faces shown are : oo P, P 3o , P oo, OP, and 2P ,as indi- 

 cated. From all the outlines and from basal cleavage or irregular 

 fissures run dark lines, in uniform direction for each individual of 

 the twin, and penetrating varying distances into the crystal. This 

 undoubtedly represents an incipient stage of that parting, which, 

 ' the large crystals of the rock, occasions the brilliant luster, for 

 t hrse dark lines do not represent ueedles of any mineral substance, 

 but the air films filling the fissures. 



This parting may be seen upon all microscopic sauidine crystals of the rock, and 

 even the irregular grains of that mineral in the grouudmass, when cut in the right 

 direction, show a very fine, delicate striation, which is undoubtedly due to the same 

 cause. As seen from the figure, the position of the surface is that of a positive hemi- 

 orthodome, for the cleavage plates of large crystals show the plane to be at right angles 

 to the clinopinacoid. Assuming the axial ratio 



a : I : c = 0.653 : 1 : 0.552 and ft = 64, 



as determined by Striiver, 1 for free crystals of sanidine, the face corresponds closely to 



J/-P*. This would require an angle of ^2 40' with the basal plane, while that 



'Cited by Tscbermak, Lehrbucli dcr Mineralogie, p. 455, 1*3. * 



