14 TJie A)jicricaii Geologist. January, iwts 
General Characters of the Feldspars. 
The feldspars are all closely related as to form, and their 
chemical composition varies from one to the other according" 
to the prevalence of the alkaline bases. They are silicates of 
alumina with an alkaline base, and hence are colorless. 
For? US of the Feldspar's. 
Orthoclase, microcline and anorthoclase may be associated 
in one group on account of their near identity of form and the 
similarit}" of theirbases. Orthoclase is monoclinic*, and anor- 
thoclase scarcely varies from monoclinic. Figures i, 2 and 3 
represent common, simple forms of orthoclase. The angle 
001 A QIC is 90". In anorthoclase it is practically 90°. In 
the upper positive quadrant of the crystal it is a little more 
and on the negative quadrant a little less than 90°. In the 
lower right quadrant it is less and in the lower left quadrant it 
is more than 90°. In microcline and anorthoclase, therefore, 
as with the plagioclases, the basal plane of the crystal in its 
conventional position, i. e., with the vertical axis(r) perpen- 
dicular, tips not only forw^ard toward the observer, but slightly 
toward the right. The angle 001 A 100 is ii6°7' in ortho- 
clase and microcline, and ii6°22' in anorthoclase. 
Orthoclase is frequently elongated in the plane of sym- 
metry, that is, parallel to the face 010, in the direction of the 
horizontal axis (a), the crystals then taking the forms of quad- 
ratic prisms, the real prism faces no and no being reduced to 
comparatively insignificant dimensions (fig. 3). Anorthoclase 
is often elongated parallel to the edge 110:110. 
The plagioclases are distinctly triclinic, yet the angle (y 
does not depart far from 90°. Their forms therefore are quite 
near that of orthoclase. The obtuse angle <* (ooi A 010) 
in the plagioclases is as follows: albite, 93°36'; oligoclase. 
93°5o'; andesine, 93°(?); labradorite, 93°2o'; anorthite, 94° 10'. 
By the development of the faces 001 and 010 they are sub- 
jected to the same elongation as orthoclase (fig. 3), and in 
addition they are sometimes elongated parallel to the edge 
001:100. This elongation produces the variety pericline of 
albite (fig. 15), and when twinned gives rise to pericline stria- 
tions which appear in all the plag"ioclases on 100 and 010. 
*According to Mallard orthoclase is triclinic. 
