3 io BRITISH PIKRITES. 



These dolerites in Scotland present no differences froni those of Ter- 

 tiary age. The rock has usually a dark-grey speckled character, and 

 seldom contains orthoclase or any original quartz. Sometimes the 

 ground mass is glassy, with dark trichites and microliths. The felspar 

 is probably labradorite, often contains minute particles of glass, and 

 may be studded with apatite. The augite is usually fractured, as in 

 the diabases ; the apparent fracturing being due to included triclinic 

 felspar. Oiivine is rarely recognisable. Dr. Geikie has adduced 

 evidence to show that the felspar crystals were already formed when 

 the rock was in a fluid state. 



The basalts, as usual, are the finer-grained dolerites, and are well 

 seen in the rocks of Craiglockhart Hill and Long Row, near Edin- 

 burgh. The augite crystals are nearly unbroken ; olivine is almost 

 always visible. Octahedrons of magnetite occur, but apatite needles 

 are rare. At Mid-Tartraven in Linlithgowshire the olivine crystals 

 abound in magnetite. 



Pikrite. Another group of rocks characterised as serpentine- 

 olivine is represented by pikrite. In Scotland it is only known from 

 Blackburn, near Bathgate, and the island of Inchcolm. It formed 

 a true lava-sheet at Blackburn, but appears to be intrusive at Inch- 

 colia. It is blackish-green, and contains olivine and augite, with brown 

 biotite, and has the crystals often united together with serpentine. 



Pikrite is a rare rock in Britain. It occurs in situ at Little 

 Knott, east of Bassenthwaite. 1 Boulders of a similar rock have 

 been described by Professor Bonney from near Pen-y-Carnisiog 

 in Anglesey. This, like the pikrites of Fifeshire, described by 

 Professor Archibald Geikie, 2 is characterised by a ground mass 

 formed of small tufts of needle or blade-shaped crystals of horn- 

 blende, which from its optical properties resembles actinolite, and 

 is not regarded as an original constituent of the rock. Secondly, 

 there are both small and large green-coloured and strongly dichroic 

 hornblende crystals. Augite is found in colourless grains and 

 crystals, which are usually embedded in a chloritic mineral. 

 Opacite and rounded grains resembling magnetite occur with many 

 pseudomorphic constituents, which Professor Bonney believes to indi- 

 cate the former presence of olivine. The larger hornblende crystals 

 are of a brown colour, and are believed to have been originally augite. 

 In the Fifeshire rocks olivine still remains as the dominant mineral 

 well preserved. 



Porphyrites. Many of the rocks which were formerly mapped as 

 felstones and porphyrites, Professor Geikie groups as felspar mag- 

 netites, and distinguishes from felstones under the name porphyrites. 

 They agree with the Old Red Sandstone lavas, and are among the 

 thickest and most widespread of the Carboniferous period, extending 

 through Berwick, Roxburgh, and Dumfries, and through the Garlton 

 Hills of Haddington, are seen at Calton Hill and Arthur's Seat, and 

 range through Dumbartonshire, Renfrewshire, and the north of Ayr- 



1 Bonney on Hornblende Pikrite, Q. J. G. S., vol. xxxvii. p. 137. 

 3 Trans. Roy. Soc. Edin., vol. xxix. 





