236 HE. H. H. THOMAS OX XEXOLITHIC [vol. Ixxviii, 



cent, of silica furnishes a very natui-al acid limit to the tholeiites. 

 Beyond that one meets with the andesitic rocks already alluded to 

 as leidleites and inninmorites, and also other types distinguished 

 by the degree of crystallization. The Ehud" a"Chromain tholeiitic 

 margins have a very close analogy to leidleite, as shown by their 

 tendency to acicular and variolitic stiiicture, and this agrees with 

 the silica percentage shown in Analysis II. In the Geological 

 Survey Memoir on the ' Geology of Colonsay & Oronsay ^vith part 

 of the Eoss of Mull ' 1911, p." 92, the xenolithic sheet of Ti-aigh 

 Bhan na Sgm-i-a is described by Dr. Flett as a dolerite of the 

 olivine-free type, and is exactly comparable with the basic margins 

 of the Eudh" a'Chromain sill. 



The composition of the stony portion of the Eudh" a"Chromain 

 sill is recorded in Analysis II, by Mj.-. Eadley. Compared with 

 normal andesites the silica percentage is relatively low, and there 

 are other divergences on the basalt side. 



AXALTSES OF XEyOLITHIC SllLS A>'D XEyOLITHS. 



I. 11. III. IV. 



SiO.^ 





Per cent. 

 70-70 



Per cent. 



53-97 

 1-24 



14-65 

 3-62 

 6-32 

 0-30 



nt. fd. 

 0-04 

 7-98 

 4-49 

 1-52 

 2-54 

 trace 

 1-92 

 0-94 

 0-27 

 0-09 

 0-51 



Per cent. 



38-67 

 0-90 



37-27 

 2-78 

 5-07 

 0-17 



nt. fd. 



465 

 2-35 

 3-01 

 3-63 

 nt. fd. 

 0-22 

 1-63 



Per cent 

 49-74 



TiOo 





1-27 



1-49 



AI2O3 



FeoOs . . 





11-78 

 1-32 



34-99 * 

 1-53 



FeO 



MnO 



(CoXi)O . 





3-45 



007 



0-34 



0-15 



nt. fd. 



BaO 



CaO 



M-0 



K.^0 





...'. 1-30 

 0-53 

 4-71 



nt.fd. 

 0-88 

 0-66 

 1-72 



Xa-^O 



Li,b 





2-48 



nt.fd. 



3-76 

 trace 



H-0 at 105 



= C. 



0-50 



0-61 



H.-O above 

 P.'05 



105° C 



ri4 



0-26 



3-44 

 0-86 



FeS-^ 



CO9 





." 0-51 



nt. fd. 

 0-04 



S 





0-08 









Totals 









100-10 



100-40 



100-35 



100-21 



I. Felsite related to iuninniorite. Acid sill from an exposure soutli of Coire 

 Buidlie, about balf a mile north of Carsaig (S. 18464) : "' Summarr of 

 Progress for 1915 ' Mem. Geol. Surv. 1916, pT26 (Anal. F. E. Ennos).' 

 II. Tholeiite. Basic portion of composite intrusion, Eudh' a'Chromain, west 

 side of Carsaig Bay (S. 17170) ; "' Summary of Progress for 1914 ' Mem. 

 Geol. SuTT. 1915, pp. 56, 57 (Anal. E. &. Eadley). 



III. Xenolitb. containing basic plagioclase, sillimanite, sapphire, and spinel 



(,S. 16612 ), from the basic portion of the sill, Xuns" Pass, Carsaig ; 

 ' Summarv of Proarress for 1913 ' Mem. Geol. Surv. 1914. pp. 80, 81 

 (Anal. E.G. Eadley). 



IV. SiUimanite-buchite. Xenolith from the basic portion of a sill, Xuns' Pass, 



Carsaig; *0"48percent. of the alumina is present as corundum, 'Summary 

 of Prosrress for 1914' Mem. Geol. Surv. 1915. pp. 56. 57 (Anal. E. G. 

 Eadleyt 



The majority of the xenolithic sills are not markedly composite. 



