

DC, 



AUG1TE. IIOBHBLBKDK.W 



Si0 2 ... ... 44-22 ... ... 3G- ( J1 



A1 2 3 ... ... 10-40 ... ... 1G-30 



Fe,0 8 ... ... 11-98 ... ... ;V2K 



FeO ... 5-77 ... ... 12-27 



MgO ... ... 7-02 ... ... 8-83 



CaO 22-54 1G-91 



102-02 96-50 



It must be remembered that these two minerals have the same appearances 

 and relations as those of the Inehcolm picritc. The analyses prove that they 

 are essentially different in chemical composition. 



The biotite does not appear to occur in well formed crystals. It is a 

 deep brown in basal sections, and strongly pleoehroic in sections parallel to 

 the vertical axis. It is usually associated Avith secondary products and may 

 itself be Avholly or in part of secondary origin. RoHEBACH mentions the 

 occurrence of biotite as a secondary product after augite in the teschenites. 



Felspar varies in amount in different specimens, but is usually 

 present only in very small quantity. It occurs in forms which give lath- 

 shaped sections and also as irregular plates. Twin lamellation is sometimes 

 preserved, and as the extinctions on both sides of the trace of the twinning 

 plane are often very high the species probably approaches anorthite in 

 composition. A striking feature in the felspar of this rock and also of those 

 in a diabase from the neighbouring island of Car Craig is the frequency with 

 Avhich they are represented by xeolitic pseudomorphs. Whether the 

 conditions haA'e been peculiarly favourable to the decomposition of felspar ; 

 or Avhether the felspars have been peculiarly liable to alteration, is 

 perhaps difficult to determine. The fact that they Avere probably allied 

 to auorthite in composition suggests the latter view. 



Titaniferous iron ore is present, and sometimes shoAvs traces of alteration 

 to leucoxene. 



Apatite occurs sparingly. The secondary products are yellow and green 

 serpentine, chlorite and zeolites. 



Another variety of picrite allied to that of Inchcolm has been described 

 by Dr. GEIKIE from Blackburn, near Bathgate. It forms a part of a lava 

 stream " Avhich Avas erupted and solidified at the surface during the 

 accumulation of the older part of the carboniferous limestone series of West 

 Lothian." In the Blackburn quarry a- marked difference exists between the 

 upper and loAver portions of the mass. The former contains much felspar 

 and is simply an altered olivine-dolerite (diabase) ; the latter contains very 

 little felspar and is an altered picrite. It is traversed by " veins of 

 serpentine and chrysotile, sometimes six inches thick and often streaked Avith 

 calcite." This portion has been Avorked for many years as lakestone : a 

 material which is used for the soles of ovens. Under the microscope this 



(2) Alkalies wore detected but not determined. A part of the deficiency is thus accounted 

 for. 



