VOLCANIC ROCKS OF EAST LOTHIAN. 



119 



Blackie Heughs is similar to that of Hailes Castle, in the character of its ground-mass, 

 but it differs from that rock by the absence of porphyritic crystals of felspar. On the 

 other hand, there is a greater amount of olivine crystals, with which some porphyritic 

 grains of augite are associated. 



The chemical composition of the Hailes Castle rock is represented in the following 

 analysis, made for the Geological Survey by Mr J. S. Grant- Wilson : — 



Si0 2 . 













49-07 



A1 2 3 . 





. 







19-43 



Fe 2 3 . 











10-58 



FeO . 













2-35 



MnO . 













•32 



CaO. . 













7-87 



MgO . 













4-36 



K 2 . 













■98 



Na 2 . 













331 



H 2 . 













2-26 





100-53 







Sp. 



G. 276 









The " Ldbradorite- Basalt " of Markle Quarry* — This is a still more felspathic 

 type. In this rock olivine only occurs in small sporadic grains, while plagioclase felspar 

 is present in numerous large porphyritic crystals lying in a ground-mass of laths, 

 microlites, and granules of felspar, together with dispersed magnetite and probably 

 augite, the latter mineral being obscured by a ferruginous product of decay. 



This rock, which must be classed as a felspathic basalt ("labradorite"), has also been 

 analysed by Mr Wilson with the following result : — 



Si0 2 . 













49-54 



A1 2 3 . 













22-23 



Fe 2 3 . 













9-55 



FeO . 

 MnO . 













112 



•08 



CaO . 













719 



MgO . 













2-80 



K 2 . 













1-81 



Na 2 . 













4-56 



H 2 . 













2.42 



10130 



Sp. G. 2-7. 



II. The Upper, more Acid Lavas (Trachytes). 



The Porphyritic Trachytes are compact rocks of pale grey, buff, and brown colours, 

 and are chiefly characterised by the presence of clear glancing crystals of unstriped 

 felspar. These crystals vary considerably in the different flows, both in regard to size 

 and abundance. They are largest and most plentifully developed in the rock quarried 

 at Peppercraig, close to the town of Haddington. This rock consists of clear felspar 

 crystals, sometimes as much as 10 mm. in length, and a dark microcrystalline ground- 



* No. 629 of the Survey Collection. 



