MUSEUM OF COMPARATIVE ZOOLOGY. 181 
ning in many of the grains, and was evidently the glassy feldspar with 
these properties described as occurring in the sections of the rock in 
little areas associated with the larger clastic feldspars, or in independent 
flattened areas like those of the albite. Another portion of the micro- 
cline contained fluid inclusions, mica or kaolin flakes, and masses of iron 
oxide, and seemed therefore to represent the clastic feldspar. Ortho- 
clase was not identified. 
The interpretation of these facts is not easy, and they do not seem to 
the writer quite parallel to the cases of feldspar enlargement heretofore 
described. 
Van Hise? described an enlargement of clastic feldspars in certain 
Keweenawan sandstones in which the original orthoclase or plagioclase 
grain, charaterized by its kaolinization and a border of ferrite following 
the original rounded shape of the grain, was surrounded by a zone of 
clear feldspar with ragged outer edge, which extinguished with the core, 
and in which twinning bands were continued when present. The new 
feldspar was therefore crystallographically co-ordinated with the old. 
In the numerous cases of feldspar enlargement in eruptive rocks 
described by several writers, the new feldspar sometimes extinguishes 
with the old, sometimes does not, and appears then to be a more or less 
distinct variety. Professor Judd has lately described feldspar enlarge- 
ments? in a “ labradorite-andesite,” which he believes to have been formed 
after the consolidation of the rock and its alteration by weathering, the 
new feldspar having formed through the alteration of the glassy base. 
The original labradorite grain is surrounded by a clear feldspar fringe 
across which the twinning planes of the core are prolonged, but in which 
the optical constants have a different orientation, appearing to belong 
to a more acid feldspar. Tongues of this feldspar sometimes pene- 
trate the old core, which is kaolinized. 
Van Hise mentions the fact,® that in the mica gneisses of the Black 
Hills the only microscopic clue to clastic origin of the rocks is found in 
the presence of particles of iron oxide in the outer portions of the (en- 
larged) quartz grains, but that this is evenly distributed through the 
feldspar, which hence has entirely formed in place. 
The feldspars described in the present paper seem to represent both 
' this completed stage, and intermediate stages in which more or less 
original clastic feldspar remains. 
1 Am. Journ. Sci., Vol. XX VII. p. 399. 
2 Quart. Journ. Geol. Soc., Vol. XLV. pp. 175-186. 
3 “Pre-Cambrian Rocks of the Black Hills,” Bull. Geol. Soc. America, Vol. IL 
p. 227. 
