G. W. Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 363 



especially in regard to the complex pyroxenes, amphiboles, and 

 micas, are rarely available. It is generally necessary to make certain 

 assumptions as to the chemical composition or amount of one or more 

 minerals present in the rock, and to test the correctness of these 

 assumptions by the congruity of the result with what is known or 

 reasonably conjectured as to the quantitative mineral composition of 

 the rock. In the present case, for example, the exact chemical 

 composition of the olivine, titanaugite, and barkevikite present in the 

 rocks is not known. There is available, however, an analysis of 

 barkevikite 1 from the lugarite of Lugar, by Mr. A. Scott, and it is 

 a reasonable assumption, considering the similarity of the rocks, that 

 the barkevikite of the Barshaw rock has a composition similar to 

 that of the Lugar mineral. There is also an analysis of titanaugite 2 



TABLE II. 

 Modes obtained by Bosiwal Micrometeic Analysis. 



Summation 100 in each case. 



I. Lugarite, variety 1. Barshaw. 



II. Lugarite, variety 2. Barshaw. 



III. Lugarite, variety 2. Barshaw. 



IV. Lugarite, variety 3. Barshaw. 



Duplicate measurement. 



V. Lugarite, type-rock from Lugar sill. 

 VI. Bekinkinite. Barshaw. 



from the porpbyritic essexite of Crawfordjohn, Lanarkshire, also 

 made by Mr. Scott. The optical properties of this mineral are very 

 similar to those of the titanaugite of the Barshaw rocks, and the 

 chemical composition of the Crawfordjohn essexite cannot be so very 

 different from that of the Barshaw bekinkinite. Hence it is legitimate, 

 at least in a first trial, to use the Crawfordjohn analysis in calculating 

 the composition of the Barshaw rock. Actually this analysis gave 

 a good result for the Barshaw lugarite, but not for the bekinkinite. 



3 A. Scott, "Barkevikite from Lugar " : Min. Mag., xvii, p. 140, 1914. 



2 Si 2 , 42-65; Ti0 2 , 4-17; A1 2 3 , 7-47; Fe 2 O s , 4-15; Fe 0,9-98; MgO, 

 11-70; CaO, 13-75 ; Na 2 0,4-90; K 2 0,tr. ; P 2 5 , 1-38; H 2 O + -04; MnO, 

 tr. ; total, 100-19. The A1 2 3 contains some thoria, probably present as 

 inclusions of a thoria-bearing mineral. The P 2 O s probably represents 

 inclusions of apatite. 



