348 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



No occurrence of topaz iu 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 erujitive rocks are said to occur, but the counectiou between 

 the two has not been deujoustrated. 



The satin-like luster of the sanidines. — The lustrous Surface is iu the oithodiagoual 

 zone and inclined a few degrees to the orthopinacoid, as is evident in the Carlsbad 

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

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

 parallel to the surface of luster, and with a knife-blade Hakes 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 flne striation at right angles to the line of oo Poo 

 and ± to the directions of extinctiou. Thin flakes split off parallel to tbe lustrous 

 surface show, under the micioscope, that the luster is due to interference of light in 

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

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

 colors of interference, while those composed of more plates are 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 tiims near the surface and to its 

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

 be seen upon the lustrous surface of the (srystals. 



In the drusy cavities above described the sanidines 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, afford 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. Thefacesshownare:ccP, «P5o,Pa., OP, and 2Pcc ,as indi- 

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

 fissures run dark Hues, in uniform direction for each individnal of 

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

 undoubtedly represents an incipient stage of that partiug, which, 

 Fio.i.-SanWinefroraN..- '" the large crystals of the rock, occasions tbe brilliant luster, for 

 vaiiit... these dark lines do not represent needles of any mineral substance, 



but the air tilms filling the fissures. 



This parting may be seen upon all microscopic sanidine crystals of the rock, aud 

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

 direction, show a very flne, 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 



« : i : c = 0.053 : 1 : 0.552 aud fi = U°, • 



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

 ^p-F-y.. This would require an angle of 72° 40' with the basal plane, while that 



'Citi'd by Tseliermak, Lcbrbiich iUt Miueralogio, p. 455, iyt3. 



