TITLES AND ABSTRACTS OF TAPERS 141 



lu this paper tlio origin and si^'iiificanco of tho above mentioned features 

 of the anorthosite-jrabbro are discussed, 



Kead from maiuiseript. witli lantern slide illustration. 



Discussed by Messrs. X. L. Bowen, C. X. Fenner, and W. G. Foye. 



EXPERlME\rS ILLUSTRATJSG DEVELOl'MEXT OF IGXEOUS HOCK TE.XTURES 



BY FKED E. WRIGHT 



{Ah.^tract) 



Tlie crystallization of certain low melting organic compounds, such as 

 thymol, menthol, guiacol, benzophenon. and others, can be readily studied 

 under the petrographic microscope during the cooling down of the melted mix- 

 tures. The order of crystallization of the several components, the crystalliza- 

 tion of the eutectic mixtures, and other attendant phenomena can be wit- 

 nesseil in process of formation. Experiments of this kind, which illustrate 

 the development of textures analogous to those found in rocks, should be of 

 educational value. 



Presented by title by request of the author. 



FELDSPARS AS IXDICATORS OF SEDIMEXTARY OR IGNEOUS ORIGIN OF 

 GNEISSES AND SCHISTS ^ 



BY EDWARD STEIDTMANN 



CONTENTS 



I'age 

 Facts on which the use of feldspars as indicators of the sedimentary or igneoxis 



origin of the parent materials of gneisses and schists is based 141 



How feldspars may be used sis indicators of the parent materials of gneisses and 



schists 143 



Application of the feldspar test to the Coutchiching schist south of Rainy Lake, 



Minnesota 143 



Facts on whk ii the Use of Feldspaks as I^-DlCATORS of the sedimentary 



OR iG.NEors Origin of the parent Materials of Gneisses 



AND Schists is based 



Feldspars appear t(. have value as indicators of the sedimentary or igneous 

 (»rigin of the parent materials of certain gneisses and schists, in view of the 

 following considerations : 



In an igneous rock the plagioclase fel(lsi)ais represent a nairow i:nig(> of 

 composition. Acid and basic plagioclase feldspars rarely occur inteiniingled 

 in an igne«.us rock, excer»ting in certain zonal growths. Orthoclase generally 

 c»ccurs with acid plagioclase, rarely with basic varieties. The feldspars of 

 sediments, however, are not controlled by the laws governing those of igneous 

 rocks. Mixture of all kinds of feldsr»ars are possible in sedimentary, but not 

 In igne<.us rocks. So generally recognized is this law of association (.f feld- 

 spars In I^eous rocks that it forms the basis of one of the best known methods 



' ManiiHcript received by the Secretary of the Society Decern Ixr l'», linf). 



