362 G. W. Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 



calculation from the chemical analyses. In making the Rosiwal 

 measurements it was not found possible to treat the leucocratic 

 minerals separately, as their boundaries were so indefinite and 

 obscured by alteration. Hence they were measured as a group of 

 colourless minerals (groundmass) as against the easily measured 

 coloured minerals. To arrive at the mass proportions of the con- 

 stituents from the data thus obtained, it was necessary to know the 

 specific gravity of the groundmass. This was obtained by calculation, 

 knowing the specific gravity of the rock as a whole, and the specific 

 gravities and volume proportions of the coloured minerals. These 

 measurements do not give the complete mineral composition, but they 

 provide information as to the relative quantities of the coloured 

 minerals, and the ratio between the light (felsic) and dark (mafic) 

 constituents. 



TABLE I. 



II. 



III. 



Bekinkinite, 1 III, (5) 6.4. (4) 5 (Papenoose). Barshaw, near Paisley. 



Analyst, E. G. Badley. Quoted from Geology of the Glasgow District 



(Mem. Geol. Surv.), 1911, p. 134. 

 Lugarite, II, (6) 7 . 1 . (4) 5. Lugar, Ayrshire. Analyst, A. Scott, M.A., 



B.Sc. Analysis published for the first time. 

 Lugarite, II, 6 (7) . 1 . (4) 5. Barshaw, near Paisley. Analyst, A. Scott. 



Analysis published for the first time. 



In order to calculate the quantitative mineral composition from 

 the chemical analysis of a rock it is necessary to know the exact 

 chemical composition of the constituent minerals. These data, 



1 The figures following the name of the rock show the exact position of the 

 rock in the American Quantitative Classification. See Cross, Iddings, Pirsson, 

 and Washington, Journ. Geol., xx, pp. 550-61, 1912. 



