522 PROCEEDINGS OF THE ROCHESTER MEETING 



these intersect at narrow angles. The}' are in two groups, apparently related to 

 strike and dip. Some veins are interrupted without continuation, others with 

 continuation en echelon. 



The paper was discussed by J. E. Wolff, B. K. Emerson, Bailey Willis 

 N. H. Winchell, A. C. Lane, and the author. 



The last paper of the session was the following : 



REGENERATION OF CLASTIC FELDSPAR 

 BY N. H. WINCHELL, 



Already several American geologists have noted enlargements of feldspar crystals 

 and fragments of crystals in clastic strata, and some have described most of the 

 phenomena to which I desire to call attention. Perhaps the earliest of these was 

 Irving, who, in association with Van Hise, in 1884 described quartz and feldspar 

 enlargements in various sandstones in the Lake Superior region.* Professor Van 

 Hise gave what appears to be the first description of feldspar enlargements in 

 American rocks. He illustrated the extension of albite twinning bands in a plagio- 

 clase fragment into the new rim formed in a Keweenawan sandstone underlying a 

 sheet of diabase. 



The next description of similar phenomena in American literature seems to have 

 been that of Dr E. Haworth,| but in this case the original feldspar nuclei were not 

 recognized as clastic, but the enlargements were supposed to have been formed on 

 the outer surfaces of crystals after solidification from a liquid magma before final 

 cooling. Doctor Haworth remarks that "of course this kind of enlargement is 

 entirely different from those in fragmental rocks described by Irving and Van 

 Hise for quartz, and by Van Hise for orthoclase and hornblende." He does not, 

 however, point out any difference, except that in the Missouri granite the rock is 

 massive and was once nearly or quite molten, whereas in the case of the Kewee- 

 nawan sandstone the rock is plainly clastic. This constitutes, it must be ad- 

 mitted, a difference in the rocks in which the phenomena are seen. The actual 

 phenomena of the enlargements, however, are quite identical with phenomena 

 described since in admittedly clastic rocks. 



In 1891 Dr J. E. Wolff gave a lucid exposition of the metamorphism of clastic 

 feldspar in a clastic rock in western New England, calling attention to some of 

 the same phenomena as mentioned by Haworth. J He found that new albite and 

 new microcline are formed about original clastic fragments, and that the enlarge- 

 ments are fresh and glossy while the nuclei are clouded by great numbers of 

 minute inclusions. He suggests that the so-called porphyritic albite crystals found 

 in the schist overlying this conglomerate may be due to an original replacement of 

 some other feldspar fragment by albite material, and that the growth of the albite 

 crystal after total replacement of the fragment was continued beyond the limits 

 of the original grain, leaving some of the iron products of such transformation in 



♦Secondary enlargements of mineral fragments in certain rocks. Bull. U. S. Geol. Survey, 

 no.8, L884. 

 % A contribution to the Archean geology of Missouri. American Geologist, May and June, 1888. 

 JMetamorphism of clastic feldspar in conglomerate schist. Bull. Mus. Comp. Zool., vol. 16, 173. 



