Prazer.] «'o [Dec. 20, 



The modifications of light at a few points seemed to be caused by im- 

 purities. 



A small rhomb examined in the Fuss instrument, under a magnifying 

 power of 27.") diameters gave : 



Acute angle (imperfect) 73° 30 



Obtuse angle " 



73'30M 

 98° 30') 



A profusion of small black specks distinguished the face of this crystal 

 which was apparently therefore not homogeneous. It showed four ex- 

 tinctions. 



A third rhombus at 275 diameters showed : 



Acute angle 72° ) 



Obtuse angle 101° ) 



Neither of the above angles was perfect. 

 This crystal also showed four extinctions. 



The ground mass is composed of minute objects, often stellate in struc- 

 ture, among which are probably small columnar masses of Rutile, as the ti- 

 tanic oxide in the analysis suggests. They are exceedingly small and, ex- 

 cept here and there, do not indicate any definite order or arrangement. 



This ground mass is not certainly determined. The tufts are sometimes 

 flat and frayed at the ends like a piece of worn cloth. The}^ polarize 

 feebly within different shades of brown. 



The following is an analysis of this trap by Dr. Genth. 



Silica (Si0 2 ) 50.79 



Titanic Oxide (Ti(),t 0.70 



Phosphoric Oxide (P 2 5 ) 0.15 



Alumina (A1 2 3 ) 14. 19 



Iron Sesqui-oxide (Fe 2 O a ) 3.84 



Ferrous Oxide (FeO) 7.44 



Manganous Oxide (MnO) 0.48 



Lime (CaO) 9.75 



Magnesia (MgO) 7.88 



Potash (K,0). 0.95 



Soda ( Na 2 6) 1 .89 



Ignition 1.95 



Total 100. 01 



Herewith are presented, side by side, average analyses of Labradorite and 

 of Pyroxene both calculated from the data given in the last edition of 

 Dana's System of Mineralogy (Fifth Edition, 1877). The former is based 

 upon forty analyses of Labradorite and the latter on eighteen analyses of 

 Pyroxene from eruptive rocks. 



