PLATE XXIV. 



FIG. I. 

 ANDESITIC BASALT. 



ESKDALEMUIR, DUMFRIES. 



Magnified 45 diameters. 



The substances represented are felspar (10), augite (7), magnetite and brown glass. 



The felspars occur as simple individuals or binary twins. In some cases they show 

 multiple twinning. The augite occurs in small granules, which sometimes show a 

 decided approach to crystalline form. They are nearly colourless. The brown glass is 

 perfectly isotropic. The colouring matter is not, however, uniformly distributed. The 

 magnetite and augite are frequently surrounded by a nearly colourless space, as if the 

 formation of these minerals had been accompanied by the abstraction of the colouring 

 matter from the glass. The rock is vitreous in aspect. It has been described by Dr. 

 GEIKIE (Proc. Hoy. Pliy. Soc., Iv.lin., vol. V., 1880). 



FIG. II. 

 BASALT. 



TAXtiLEY HILL, DUDLEY, STAFFORDSHIRE. 



Magnified 200 diameters. 



The minerals represented are magnetite, minute augite crystals and felspar 

 microlites. There may be also a little colourless glass. 



This slide represents the character of the ground-mass of those basalts which 

 contain minute crystals as distinguished from those which contain only granules of augite. 

 The ultimate ground-mass is essentially felspatbic, and under crossed nicols splits 

 up into an aggregate of extremely minute felspar microlites and plates. It is very difficult 

 to be certain as to the existence of any isotropic interstitial matter. If it exists it is perfectly 

 colourless, like the felspar, and present only in very small quantity. This type of 

 ground-mass is not known at present from the Tertiary Basalts of the Brito-Icelandic 

 province, but is extremely common in the Tertiary Basalts of the Continent (e.y., the 

 Bohemian Basalts), and in the carboniferous Basalts of the Midland A'alley of Scotland, 

 and the neighbourhood of Limerick, in Ireland. In many cases (e.g. , rock near 

 Dunsapie Loch, Edinburgh, and Craig of Neilston, Renfrewshire) the microlites of 

 augite are much longer in proportion to their breadth than in this figure. 



The rock of Tansley Hill is rendered micro-porphyritic by the occurrence of larger 

 felspars than any represented in the figure. It will be observed that in this figure a 

 very high magnifying power has been used. 



