BIOTITE TINGUAITE DYKE UOCK. 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 i)roduct 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^AlaSieOic ^ 



16.68 K,AL Si„ Oio > 67.28 % feldspar. 



3.44 CaAl, Si, Os ) 



9.61 NaeK2Al«SiA4 

 8.09 H, AL Si2 O,, 



2.62 SiO, 



6.00 Na^ Fe, Si, Oi, = G.OO % a?girine. 



2.90 Biotite. 



3.50 Fe, O3 (Fe Mn) O I ^'^^ % . ^'""^^^ ^"^ ^^-- 



> netite. 



20.32 % nepheline, kaolin and 

 quartz. 



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. 



