TITLES AM) ABSTRACTS OF PAPERS 14?) 



laws of Msso«iation as the IVUlspars of an iu'ueous rock. Kxtonsive feldspathic 

 60«liinoiitary (lo|K)sits are pMiorally derived from many kinds of rocks: hence 

 tlieir fehlspars can not he expected to show the same characteristic association 

 «.s those of igneous nuks. 



2. RecrystallizatioM of non-fel«lspathic materials, muds, etcetera. If oriixi- 

 nal material was nniform in comi'ositiou. it is proi)al)le that the fehlspars 

 ■developetl may show tlu' limited ram^e of composition of those which occur 

 in igneous nu-ks. The contrary, however, is to he expected in most cases. 



'A. The feldspars niay have heen introduce<l hy magmatie or other thermal 

 solutions l)ofore schistitieation. This may or may not cause a violation of the 

 rules iroverning the asso<'iation of feldspars in igneous rocks. Such an intro- 

 iluetion may he exptn-ted to he local rather than regional. Its local secondary 

 <'liaraeter miglit he recoi:ni/ed if loi>ked for. 



How Fkidspaks may \u isku as Indicators of thh parent Matkriat.s of 

 Gnkissks and Schists 



The conclusions to he drawn from the preceding general considerations are: 



1. Schists and gneisses whose feldspars show the same laws of association 

 «s those of igneous rocks are prohaldy of igiu^)us origin. The conclusion is 

 strengthened if the rocks in (luestion are of great areal extent and thickness. 



2. Schists and gneisses whose feldspars do not show the laws of association 

 I)eculiar to igneous rocks are almost certainly of sedimentary origin. Great 

 areal extent and thickness of the rocks in question add to the validity of the 

 conclusion. 



Detaile<l studies of thin sections made for the pur|)Ose of testing out the use 

 of feldspars as a method of distinguishing the sedimentary or igneous origin 

 of .schists and gneisses have heen in progress hy the writer for some time 

 and are sufficiently advanced to warrant the helief that they may prove vain 

 af>le in some ca.ses, Ohviously, the statistical method of identifying the 

 plagiiM-lase feldspars can not he used in this study, since it is hased on the 

 assumption that the composition of the [)lagioclase feldspars of a slide is 

 uniform. Instead the method of measuring the angle hetween the optic plane 

 and the trace (»f Olo in sectiims perpendicular t(> X ov Z has heen found to he 

 a very useful metho<l. 



Appr.KATioN OF tiif: I'ii-dsi'au '1'kst to tmk ('oitciiiching Schist sot th of 



K A I N ^ f . A K K, .Ml .\ N KS( )T A 



One of the roiU^ whi.h the writer has sfudic<l in considci-ahlc detail l>y this 

 nietlMHl is a tine to me<linm grained gray gneiss or schist which maintains a 

 reniarkahly uniform hahit over hundreds of square miles hetween Lake Ver- 

 milion and Kalny Lake of northern Minnesota. On Kainy Lake the planes of 

 «cliistosity of this material dip to the northward under Keewatin grecnstomvs. 



into which It appears to gra<le. the gradation.il pluise heing i lore than 



alMJUt 10 f»H»t wide, P.oth greenstone and the gray schists are intruded hy an 

 Arehean granite hatholith whose outcro|)s are more eons|>i<'Uous in the schist 

 area than in that <»f the irreenstono.. The >;ihisf ;ir»'a presents such a mixture^ 



