60 BULLETIN OF THE ESSKX INSTITUTE. 



Uiider the microscope the rock is seen to be composed 

 mainly of feldspathic laths and plates with much nepheline 

 and less amounts of segirine, magnetite and biotite. A 

 little sodalite, apatite and zircon are also present. 



The feldspar forms the principal constituent and pre- 

 dominates in lath-shaped sections, which have a ragged 

 appearance, due to frayed-out ends and a fibrous structure. 

 This fibrous appearance is caused by lamellar intergrowths 

 of the soda and potash feldspai s, inicrocline and albite, 

 forming microline-microperthite. Some of the broader 

 sections show a rather coarse intergrowth of the two feld- 

 spars giving extinctions on different parts of the same 

 section, corresponding respectively to these two feldspars, 

 while some which do not show the perthitic structure may 

 be anorthoclase. Carlsbad twinning of the laths is com- 

 mon. Besides the lathshaped sections, many plates occur, 

 which are cleavage sections parallel to M, of albite. 

 They show basal and prismatic cleavage cracks, an optic 

 axis and extinguish at 20°. 



The nepheline occurs next to the feldspars in amount, 

 and occupies the position of a filling matter in the inter- 

 spaces formed by the feldspars. It has been the last min- 

 eral to form and most of it is in xenomorphic angular 

 sections, but here and there, well defined hexagonal plates 

 are seen. The nepheline has altered and is present as 

 grayish, muddy, granulated sections which are apparently 

 mixtures of nepheline with kaolin and very fine grains of 

 quartz ; the alteration being to a hydrous aluminium 

 silicate through loss of alkalies, rather than to a zeolite. 

 The sections still retain their index above that of the 

 feldspars and gelatinize with HCl as shown l)y fuchsin 

 staining, yet this reaction was not so well and easily ob- 

 tained as with fresh nepheline. 



^o:irine is disseminated in the rock in fraffments and 



