and the Results of its Alteration. 273 
5. Microcline. 
n_the best specimen observed, the crystalline planes, bot 
prismatic and terminal, are perfectly distinct, but nothing was 
left of the original mineral. This pseudomorph consisted, for 
the most part, of the potash feldspar, but a small portion of 
one side was soda feldspar (or albite). The relation of these 
two minerals is shown in fig. 10, a case in which the albite is 
present in much larger quantities than in that described, it mak- 
ing up about half the crystal. 
- Lhe composition of the yellow granular feldspar is shown 
by the following analysis by Mr. Penfield :— 
G. == 2548, L. iH? Mean. ‘ 
Si0, 64°55 64:55 1076 6 
Al.O; 19°70 19°70 ‘191 1-07 
K,0 15°66 15°59 15°62 166) . : 
Na,O 053 0-64 0°58 wnt opp 
Ignition 0-12 0°12 0712 
100°57 
It will be seen that the above analysis corresponds very closely 
with the normal composition of microcline, K,A1,Si 
Figures 8 and 4 show further the manner in which the 
Am. Jour. 4 perme Vou. XX, No. 118.—OcrT., 1880. 
