BY W. N. BENSON. 153 



one another, but are separated by a matrix of minute equidimen- 

 sional grains of felspar, either untwinned, or twinned with ex- 

 ceedingly minute lamellae. Quartz may also occur in this mosaic, 

 bub its determination is difficult. The trachytic fragments may 

 be entirely felspar, or may be homogeneous magnetite- kerato- 

 pliyre, but generally they are keratophyre with a magnetitic 

 core, and purely felspathic outer parts. Complementary to this 

 type of rock, there is still another (1188) in which the non- 

 trachytic matrix predominates, and is grey-coloured owing to a 

 regular distribution of magnetite throughout; the inclusions are 

 felspathic trachytic keratophyre, and sometimes contain a mag- 

 netitic core. Even in this rock there is a concentration into 

 lines between the inclusions, giving a honeycomb-appearance. 

 (Plate xxvii., fig.l). 



In other rocks, the magnetite is more evenly distributed, and 

 the rock begins to take on a more slaggy or scoriaceous habit, 

 and its rough, irregular cavities are filled with calcite. After 

 having removed the carbonate by hand, as far as was possible, 

 during the rough crushing of the rock (1086), the remainder was 

 analysed, with the result given(p.l39). This confirms the optical 

 determination of the felspar as albite, for the composition of the 

 felspar, calculating from the analysis in the same way as before, 

 should be Ab^oAn^. The amount of titanium present is rather 

 less than might have been expected from an analogy with the 

 chemical characteristics of the spilitic series. The same features 

 are even more strongly developed in the very richly magnetitic 

 keratophyre shown in Plate xxvi., fig. 12. The association of 

 areas rich in magnetite with others poor in that mineral, some- 

 times merging into one another, sometimes sharply defined, and 

 the strongly marked trachytic texture, with occasional brecciated 

 structure, are also distinctive features. The rock is strongly 

 attracted by a magnet : the felspar is, apparently pure albite and 

 water-clear, but the rock is so scoriaceous, and so intimately 

 mixed with calcite, that density-determinations or chemical 

 analysis would be of little use. 



Some of the magnetite-keratophyres of this complex contain 

 more or less quartz. In one(1060) near the margin of the mass, 



