2/8 MINERALS COMPOSING FELSPAR BASALTS. 



basalt may have its felspar more or less absolutely replaced by neplie- 

 line, or by leucite, so as to be conveniently distinguished as nepheline 

 basalt or leucite basalt. The former is not necessarily free from fel- 

 spar, and may contain a little leucite ; the latter may include nepheline 

 with leucite, but rarely felspar. 



The felspar basalts are more characteristic of Western Europe ; 

 the leucite and nepheline basalts are typical of the Erzgebirge, Eifel, 

 and Italy. 



Felspar Basalts. Most basalts are felspar basalts ; the plagio- 

 clase cannot be determined with certainty, though there is rarely any 

 doubt that it is labradorite. It' occurs in long prismatic crystals, 

 which are the predominate constituent of the rock, and cross each 

 other in various directions. They show with polarised light the 

 coloured striae indicative of laminated twin structure, and occasionally 

 two sides of the crystal show blue and yellow colours. 



If the lamination is absent, the felspar is identified as orthoclase, 

 as in the dolerite of Eowley in Staffordshire, where clear structureless 

 glassy felspar also occurs. 1 



Felspars often include augite, magnetite, and apatite. The felspar 

 is sometimes partly decomposed, when the interior of the crystal is 

 turbid, or a little chlorite occurs in it ; and pseudomorphs of felspar 

 in chlorite are found at Mugdock Tunnel, near Glasgow ; Craigie Hill, 

 near Edinburgh ; Deep More in Staffordshire, and Matlock in Derby- 

 shire. 



Pseudomorphs are easily recognised under the microscope, because 

 the optic axes vary in every part of the crystal, so as to produce a play 

 of colours in all positions of the prisms. 



Augite is usually found in well-formed crystals, which have a 

 brownish tinge, and show bright colours in polarised light. It gives 

 an eight-sided section, while hornblende occurs in six-sided crystals. 

 In polarised light, augite, as a rule, shows no change of colour, though 

 in the rock at the Necropolis Hill, Glasgow, it is slightly dichroic, 

 and shows a purple tinge. Hornblende is always dichroic, and as the 

 polariser is rotated the colour changes from yellow to brown. Twin 

 crystals of augite are common, and laminated crystals are found at 

 Bowling, near Dumbarton. Minute grains of magnetite occur in 

 augite crystals ; and in the Campsie Hills, near Glasgow, the mineral 

 contains cavities in lines parallel to the sides. When augite decom- 

 poses it yields a grey fibrous substance, or a turbid granular substance, 

 and is often more or less perfectly replaced by chlorite. When horn- 

 blende is present it may be brown or black, and is distinguished from 

 the augite by its shining fracture, as at Mayen in the Eifel, many 

 places in the Westerwald, at Schima and Kostenblatt in Bohemia, 

 and in Heilenberg and Gickelsberg in Saxony. 



Olivine is one of the more conspicuous minerals in basalt, often 



forming glassy oil-green grains like drops. At Dalsmynni much of 



the ground mass of the basalt is formed of grains of olivine ; at Unkel 



on the Khine, and occasionally in the Auvergne, the olivine crystals 



1 See Allport : Carbon. Doler., Q. J. G. S., vol. xxx. 





