Eakle — Biotite tinguaite Dike from Massachusetts. 4:91 



The structure of the rock differs from that of a typical 

 tinguaite, in that the component feldspar and segirine minerals 

 <lo not occur as needles, but as much stouter lath-shapes and 

 prismatic crystals, indicating a slower rate of cooling of the 

 magma and thus producing a phase of tinguaite much less 

 dense and more noncrystalline than common. The rock is 

 classed here as a biotite-tinguaite, yet from the presence of 

 many feldspar sections tabular to M, it can as well be con- 

 sidered a phase of solvsbergite. It stands intermediate between 

 a quartz-free nepheline tinguaite and a nepheline- segirine solvs- 

 bergite ; such a rock is described by Brogger from a dike 

 between Tjose and Aklungen.* 



The analysis gives : 



Si0 2 60-05 



Ti0 2 0-11 



A1 2 3 19-97 



Fe 2 3 ._. 4-32 



FeO 1-04 



MnO 0-79 



CaO 0-91 



MgO 0-23 



K 2 3-24 



Na o 7-69 



H 2 110° 0-15 



H 2 ig. 1-26 



CI : 0-28 



100-04 



The specific gravity determined by the balance is 2*708. 



Access to the dike is very difficult, and all of the specimens 

 obtained come from near the surface and have weathered 

 enough to make it difficult to estimate, even approximately, 

 the relative proportions of the mineral contents from the 

 chemical analysis. The nepheline and biotite show the greatest 

 amount of alteration and the rock has apparently lost some of 

 its alkalies and iron from the change, through its exposed con- 

 dition. The amount of silica and alumina is more than suffi- 

 cient to combine with" the alkalies to form the alkali minerals, 

 and the excess is present in the form of kaolin and secondary 

 quartz. Fully 20 per cent of the slide appears to be nepheline, 

 yet the percentage of soda will only allow for about one-half of 

 this amount, and 14 per cent only of the rock is soluble in 

 HC1. A calculation from the percentage composition, with 

 due regard to the microscopical estimation, gives the following 

 -as an approximate mineral composition of the rock in its 

 present state : 



* Die Eruptivgesteine des Kristianiagebietes, Part I, p. 99. 



