88 On the Mieroscopieal Structure and Composition 



is also frequently collected together so as to form a dark, or even 

 black mass in the centre, the edges of which are sharply defined, and 

 correspond exactly with those of the crystal. Hexagonal crystals, 

 for example, exhibit a border filled with fine grey dust, and a central 

 portion occupied by a well-defined black hexagon ; or there is some- 

 times a black band running parallel with and at some distance from 

 the sides, the central and outer portions of the crystal being occu- 

 pied by the grey dust. With a magnifying power of 800, a portion 

 of the dust is resolved into minute granules, having a translucent 

 centre surrounded by a dark ring ; they are therefore probably glass 

 cavities. It is especially worthy of remark that this grey dust occurs 

 in precisely the same way in the nephehne of the basalts and phono- 

 htes of Tertiary age, and from widely separated locahties. The clear 

 crystals frequently exhibit faint lines parallel with the sides, and 

 they often enclose slender acicular prisms; compound and twin 

 crystals are not uncommon. 



The felspar is perfectly clear and transparent, and evidently be- 

 longs to the orthoclase group ; no striae are observable, and one or 

 two crystals give an angle of 90^ by measurement with the gonio- 

 meter. The prisms are frequently much fractured transversely to 

 the long axis, and in this respect, and in their optical character, they 

 closely resemble the sanidine of some trachytes and phonolites. 

 Twin crystals showing different colours on opposite sides of the 

 plane of junction are not uncommon. The felspar contains numerous 

 glass cavities and extremely minute crystals, which are sometimes 

 irregularly distributed, but are also frequently arranged in rows 

 parallel with the edges of the larger crystals ; the latter have also 

 caught up crystals of nepheline, and a few long slender prisms, 

 probably apatite. 



The hornblende occurs in minute green prisms, varying in length 

 from ^(5- to 2-^th of an inch, and in width from ^^^g- to xrui7 ^^ 

 They are regularly interspersed with the other constituents of the 

 base, but occasionally great numbers of them are crowded together 

 in closely compacted groups, having a nucleus of black grains of 

 magnetite ; a few prisms may also be seen imbedded in the nephe- 

 line and felspar. 



There may perhaps be some httle uncertainty whether this 

 pyroxenic mineral be hornblende or augite. I have not met with 

 any reliable angles, as most of these minute prisms appear to be 

 broken or imperfectly formed at the ends ; a few of larger size are, 

 however, distinctly dichroic, and would therefore appear to be horn- 

 blende. 



The greater part of the mass of the rock is seen to consist of 

 nepheline ; the crystals forming the base vary in size from the tItt 

 to 5^ in. across, but there are perfect hexagons which do not 

 measure more than the ?-o\rtrth of an inch ; most of them are 



