406 J. D. Dana on the Feldspar group of minerals. 
both the Unisilicate and Bisilicate type, there is some princi- 
ple on which the proportion of silica may increase without 
changing the type; secondly, that this increase accompanies 
increase in the proportion of the alkalies, (or the perissad me- 
tals) and is probably more promoted by K than Na; thirdly 
that this excess of silica occurs without a change of the type 
of the species from that of the Unisilicate or Bisilicate ; and 
that the surplus silica is either accessory silica, or part of it 
basic.* 
2. Isomorphism of the Isometric and Clinohedral Feldspars. 
Although Leucite belongs apparently to a different series in 
the Feldspar group from the clinohedral (monoclinic and tri- 
clinic) species, and its trapezohedral form has seemingly very 
little resemblance to the oblique prisms of the other Feld- 
spars, the monoclinic crystals of orthoclase and the triclinic of 
anorthite etc., are in fact nothing but distorted, or rather, 
clinohedrized dodecahedrons, variously modified by cubic, 0c- 
edral, trapezohedral and other planes. : 
In Orthoclase this clinohedrism consists (a) in giving the 
dodecahedron an obliquity in one direc- Ap. 
tion of about 8 degrees, one of its octahe- @ 
1 axes, thus inclined in a vertical plane 
parallel to a pair of dodecahedral planes, 
becoming the vertical axis; and (b) slightly 
shortening this axis ; and for the triclinic 
forms, it is nearly the same, excepting that 
_ the obliquity extends to the vertical plane 
referred to, and consequently to the planes 
about it. 
a 
The annexed figure represents a commo | 
form of Orthoclase, copied from Naumann’s : 
crystallography. The lettering corresponds to the following i 
Naumann, and Brooke and Miller: 
ee ae ee ee ae ad i, 
Naumann) 9P 2P [aPo] [aP3] ¢Pa Pm 2Pa 
; | P Zt [ M C Zz ] q ee y Oo n 
cme t ei ~ he Li ey 7 7 ra 
; ay ee z : a. 
The prismatic angle of the crystals is 118° 48’ or near vie 
and the same angle in Albite is 120° 47’; in Oligoclase 1 
42"; in Labradorite 121° 37’; in Anorthite 120° 30’. ; 
# —In these ar , this volume, p. 93, 141. fr. foot, for } read $7 - 
and 131. fr. foot, for read ¢, and for e read 6. Page 253, for RO® read R°0°F 
for SR5O read 5R3O; for Ry, Ry, Ry, read Ry, Ry, Ry; for d read ¢, and for * 
, — 
_ Fead 6. Page 255, 91. fr. foot, p. 258, 8 1. fr. top, p. 259, 21. fr. top, for dread® 
