﻿104 



ME. ft. W. TYRRELL OjS" THE 



[vol. lxxii, 



therefore, necessary to know their exact chemical composition in 

 this series of rocks before the quantitative mineral analyses could 

 be utilized. Accordingly, Dr. Scott made analyses of the barke- 

 vikite in the lugarite of Lugar, and of the titanaugite in the 

 porphyritic essexite of Crawfordjohn (Lanarkshire), minerals which 

 are optically identical with those of the same species in the Lugar 

 rocks, and occur in rocks belonging to the same petrographic 

 province. For biotite, which occurs in small quantity, an analysis 

 of a biotite from the monchiquite of Horberig, Oberbergen, 

 Kaiserstuhl, 1 was used. From a consideration of the complete 

 series of Lugar chemical analyses it is clear that the olivine also 

 varies somewhat in composition. In the peridotites and picrites it 

 is richer in the forsterite molecule than in the theralites and 

 teschenites. The average composition of the olivine in the different 

 rocks was calculated as follows : — 



Forsterite. Fayalite. 



Peridotite and picrite 4 1 



Theralite 3 1 



Teschenite 2 1 



In the same way the iron-ore was calculated to have the 

 composition 



Ilmenite : Magnetite : : 3 : 4 ; or 

 Fe 2 0 33 39-4; FeO ; 38-0 ; TiO^, 22-6. 



The chemical analyses by Dr. Scott, and the calculated Rosiwal 

 micrometric analyses are collected together in Table II, with an 

 analysis of teschenite from Mons Hill (Midlothian). 



Table II. 



Si0 2 ' 44-50 



TiOo 



Al»6, 



Fe 2 0 3 



FeO 



MnO 



MsO 



Na,0 



K 2 () 



H 2 0 + .. 

 H 2 0- .. 



P 2 0 5 



BaO(SiO) 



C0 2 



F 



FeS 2 



2-43 

 15-23 

 2-81 

 6-88 

 0-26 

 4-91 



CaO ' 12-08 



3-94 I 



2- 41 ; 



3- 09 ; 

 0-72 

 0-24 



rie 



11. 



45-26 

 301 



15-74 

 2-33 

 712 

 0-22 

 523 

 8-86 

 5-01 

 2-51 

 2-94 

 0-68 

 0-90 



p.n.d. 

 tr. 



III. 



45'55 

 2-91 

 14-89 



4- 06 

 8-23 

 tr. 

 8-03 

 6-98 



5- 07 

 1-94 



I 1-41 



0-94 



0-05 



IV. 



44-98 



3- 29 

 1585 



4- 83 

 8-11 



6- 35 



7- 24 



5- 78 

 1-05 



1-67 



0-84 



0-04 



4341 

 402 



14 r 89 

 5-63 

 9-02 



7- 01 



8- 47 

 5-63 



0- 19 



1- 21 



0-55 



Totals 



100-71 1 99-81 100-1.6 



VI. 



VII. 



46-84 



42-06 



2-06 



2-10 



16-32 



8-70 



2-65 



3-07 



7-30 



13-84 





0-07 



8-53 



16-45 



7-93 



6-97 



5-55 



4-59 



0-63 



0-28 



1-33 



1-15 



0-84 



0-75 



0-04 



0-04 



100-02 



100-07 



46-06 

 2-56 

 15-94 

 2-94 

 7-44 

 031 

 4-14 

 7-04 

 4-95 

 2-76 

 C 4-22 

 I 0-55 

 0-84 



o-io 



0-11 



6-36 



1 H. Hosenbusch, ' Elements der Gesteinslehre ' 1910, p. 300. 



