62 Ailing — Problems of Adirondack Precambrian. 



as 10% orthoclase and 20% andesine, while the sedi- 

 mentary rocks frequently exhibited a motley collection; 

 covering a much wider range. Very commonly soda- 

 orthoclase, microcline, perthite, oligoclase and labra- 

 dorite were seen in a single microscopic slide. This was 

 explained with the aid of the equilibrium diagram of the 

 orthoclase-albite-anorthite system proposed by Vogt, 27 

 Marc, 28 Becke, 29 Harker, 30 supported by the observations 

 of Day, 31 Allen, 31 and Warren. 32 If the feldspar compo- 

 sition, in the magma, was on the potash side of the 

 eutectic line the resulting crystals would be dominantly 

 the orthoclase type of feldspar, while if it was on the 

 other side plagioclase (plus a little potash feldspar) 

 would result. But if the position of the molten feldspar 

 was on or near the eutectic line the solid minerals would 

 be divided on freezing into orthoclase (carrying a little 

 soda feldspar in solid solution) and plagioclase. 

 The criteria may be summed up as follows : 



Sedimentary Origin Igneous Origin 



Original Quartz High pyroxene content 



Motley collection of feldspars Evenly "split" feldspars 



How successfully these criteria have been applied to 

 amphibolites whose origin was not forthcoming from the 

 held relations cannot as yet be stated, but the hope is 

 entertained that some progress has been made in this 

 difficult problem. 



4. The Algoman Series. 



The Anorthosite. — The anorthosite -syenite -granite 

 and gabbro series have received a great deal of attention. 

 It was Cushing 33 who first pointed out in the Saranac- 

 Long Lake Eegion that the anorthosite was the oldest 

 rock of the group, by finding dikes of the syenite cutting 

 the anorthosite. The writer can bring into court two 



27 Vogt, J H. L. : Silikatschemelzlosungen, 1914, II, pp. 120-1. 



28 Becke, F.: Tschermak, Min. Petr. Mitt. (2) 25, 106, pp. 361, 383-85, 

 1906. 



29 Marc, Eobert : Vorlesungen Tiber die Chemische Gleichewichtslehre und 

 ihre Anwendung auf die Probleme der Mineralogie, Petrographie und Geolo- 

 gie. Fig. 68, pp. 69, 111-112. 



^Harker, Alfred: Natural History of Igneous Rocks, p. 250, 1911. 

 81 Day, A. L., and Allen, E. T. : Carnegie Inst., Publ. 31, 1905. 



32 Warren, C. H.: Am. Acad. Sci., 51, No. 3, pp. 127-154, 1915. 



33 Cushing, H. P. : N. Y. State Geol., 20th Ann. Kept., p. r25-r46, 1900 ; 

 N. Y. State Mus., Bull. 95, pp. 318-322, 1905. N. Y. State Mus., Bull. 115, 

 p. 481, 1907. 



