BIOTITE TINQUAITE DYKE ROCK. 63 



all of the silica and alumina in the formation of the al- 

 kali minerals, and the excess of these two oxides must 

 evidently combine with the water to form kaolin, leav- 

 ing besides a small excess of free silica, which is seen in 

 the slide as a separation product from the alteration of 

 the nepheline. Fully twenty per cent of the slide ap- 

 pears to be nepheline, yet the soda will only allow for 

 about one-half of this amount, and fourteen per cent 

 only of the rock is soluble in HCl. A calculation from 

 the percentage composition, with due regard to the micro- 

 scopic estimation, gives the following as the approximate 

 mineral composition : 



47.16 Na^AUSieOic ^ 



16.68 K,AL Si« Oi« i 67.28 % feldspar. 



3.44 CaAl^ Si, Os ) 



s'oQR^^l'^^o''^" I 20.32% nepheline, kaolin and 

 8.Uy H, AI2 bi2 O9 > 



2.62 SiO. ) "i''^"^^- 



6.00 Na2 Fe^ Si^ Oi., = 6.00 % cegirine. 



2.90 Biotite. 



3.50Fe,O3(FeMn)O^ ^-^^ ^^ ^^^^^^e and mag- 



> netite. 



100.00 100.00 



PLATE. 



Biotite tinguaite dyke cutting augite syenite. 



The dyke may be detected near the bottom of the 

 ledge by my note book at near one end and on the other 

 by a Boston and Maine railroad time table placed in the 

 contact walls where the dyke has been eroded out. 



Salem, Aug. 21, 1898. 



