THE GNEISSES 



145 



Table Illustrating Chemical Similarity of Clastic and Crystalline 



EOCKS. 



CONSTrrUENTS 



Silica (Si02) .... 

 Titanium oxide (Ti02) . 

 Alumina (AI2O3) . . 

 Ferric oxide (Ee203) . 

 Ferrous oxide (FeO) . 

 Ferrous sulphide (FeS2) 

 Manganese oxide (MnO) 

 Lime (CaO) . . 

 Magnesia (MgO) . 

 SodaCNaaO) . . 

 Potash (K2O) . . 

 Ignition .... 



Pi 



/o 

 68.18 



* * • * 



16.20 



* » • * 



4.10 



1.75 

 0.48 

 2.88 

 6.48 





100.07 



II 



0/ 

 Jo 



61.96 



1.66 



19.73 



4.60 

 4.33 

 Trace 

 0.35 

 1.81 

 0.79 

 2.50 

 1.82 



yy.od 



dj 



III 



0/ 

 /o 



69.24 



14.85 

 2.62 



0.45 

 2.10 

 0.96 

 4.30 

 4.33 

 0.70 



m 



IV 



99.55 



0/ 

 /o 



69.94 



Not det. 

 13.15 

 2.48 



0.70 

 3.08 

 Trace 

 5.43 

 3.30 

 1.01 



99.09 





% 

 61.91 



21.73 

 4.73 



0.09 

 0.59 

 0.25 

 3.16 

 7.43 



I 



DQ 



99.89 



YI 



h 

 60.32 



23.10 

 7.05 



0.87 

 0.49 

 3.83 

 4.08 



^ 



1^ 



ft H 

 a g J3 



S •< ;5 



ir* S3 P* 



YII 



99.74 



Jo 

 65.69 



0.31 



16.23 



4.39 



IJTot det. 

 2.63 

 2.64 

 2.12 

 2.00 

 4.70 



99.71 



I. Granite: Syene, Egypt. II. Gneiss: St. Jean de Matha, Province of 

 Quebec, Canada. III. Gneiss: Trembling Mountain, Province of Quebec, 

 Canada, IV. Sandstone: Portland, Connecticut. V. Shale: England. VI. 

 Slate: Lancaster County, Pennsylvania. VII. Disintegrated granite: Dis- 

 trict of Columbia, 



in four pieces. The same features are brought out even more 

 plainly in Fig. 15, on page 143, which shows the structure of this 

 same gneiss as seen under the microscope. 



As in the present state of our knowledge it is in most cases 

 impossible to separate what may be true metamorphosed sedi- 

 mentary gneisses from those in which the foliated or banded 

 structure is in no way connected with bedding and which may 

 or may not be altered eruptives, all are grouped together here. 



Classification and Nomenclature. — The varietal distinctions 

 are based upon the character of the prevailing accessory min- 

 eral, as in the granites, forming a parallel series. We thus 

 have Motite gneiss, muscovite gneiss, iioiite-mv.scovite gneiss, 

 Jioroillende gneiss, etc. 



The name granulite or leptynite is applied to a banded quartz- 

 feldspar rock, the constituents of which occur in the form of 

 11 



