MUSEUM OF COMPARATIVE ZOOLOGY. 181 



ning iu many of tiie grains, and was evidently the glassy feldspar with 

 these properties described as occurring in the sections of the rock in 

 little areas associated witli tiie larger clastic feldspars, or in independent 

 flattened areas like those of the alliite. 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 dilferent 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 Elack 

 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 iu place. 



The feldspars described in tlie present paper seem to represent both 

 this completed stage, and intermediate stages in which more or less 

 original clastic fi'ldspar remains. 



1 Am. Journ. Sci.. Vol. XXVII. p. 309. 



2 Quart. Journ. Geol Soc, Vol. XLV. pp. 175-186. 



3 " Pre-Cambrian Rocks of the; Black Hills," Bull. Geol. Soc. America, Vol. I. 

 p. 227. 



