24 



The approximate mode calculated from 



% 



84-1 



10-7 



1-6 



1-0 



2-0 (say) 

 •5 



By allotting to chlorite, as has been done here, all the 

 oxides left over after satisfying the minerals of fixed consti- 

 tution, the following chemical composition is deduced for 

 that mineral : — 



unaltered biotite. 

 the analysis is : — 



Albite ... 

 Chlorite 

 Muscovite 

 Rutile . . . 

 Quartz . . . 

 Apatite 



SiO, 



Al,63 



Fe.Og 



Feb 



MgO 



40-4 



32-4 



3-0 



6-2 

 10'8 



7-2 



It may be that the SiO^ percentage is unduly high owing 

 to insufficient having been allowed for quartz in the rock, but- 

 even so the relative proportions of the other constituent oxides 

 show the mineral to have a very unusual composition, MgO in 

 particular being very low when compared with AI2O,. 



The abundance of rutile in the rock is reflected in 

 the relatively high percentage of Ti02, a percentage which 

 appears to be rather exceptional for such rocks, (i^^ The 

 figures for this oxide make it abundantly clear that the 

 rutile could not have been derived from the breaking down 

 of biotite, a conclusion which is in accord with the micro- 

 scopical evidence. 



This syenite is of a rather rare type, judging by the 

 small number of published analyses of similar rocks. Many 

 of the recorded occurrences are associated with basic or ultra- 

 basic rocks, and, apart from the Cape Willoughby rock 

 recorded by C. E. Tilley, only three exceptions to this rule 

 have so far come under the author's notice. The first of these 

 is the soda granite of Croghan Kinshela, Co. Wexford, 

 Ireland, described by Haughton,(i7) which Harker^is) correlates 



(ic>) The association of TiO. with sodic I'ocks has been noted 

 and commented on by various writers, although Smyth (A.J.S., 

 vol. 31, 1916, p. 37), following Washington, represents Ti as con- 

 centrating in siihalkaline rather than alkaline magmas. 



(i7)Hanghton: Q.J.G.S., vol. 12 (1865), p. 183. 



(18) Anniv. Address to Geol. Soc. of London, Q.J.G.S., vol. 73, 

 (1917), p. Ixxxiii. 



