part 2] GEANITE-GlSrEISSES OF SOrTHERN ETEE PEINUSTSULA. 93 



silliraanite, in addition to that of recognizable xenoliths o£ sedi- 

 mentary origin, confirms the view that these knots and hands are 

 xenoliths of the accidental kind. In these cases the garnet of the 

 knots themselves, and some of the garnet-grains of the enclosing 

 gneisses, are also to be considered xenolithic, for spinel and 

 sillimanite are developed within their borders. 



A som-ce for these xenoliths can be readily found in the para- 

 garnet-gneisses of the Hutchison Series, which, in common with 

 the diopside-rocks, form the shores of Sieaford Bay. 



In an earlier paper ^ I described garnet-gneisses f i-om the 

 Hutchison Series at this locality, and northwards in the hundred 

 of Hutchison, containing garnet, spmel, and sillimanite. The 

 mutual associations of these minerals (spinel and sillimanite 

 •enclosed in garnet) in the knots of the Sieaford gneisses can be 

 matched in the paragneisses of the Hutchison Series. 



It may be concluded, therefore, that, like the diopside-xenoliths at 

 Fisher}?" Bay, these garnet-spinel knots are fragments of the older 

 Hutchison Series metamorphosed and incorporated in the fluid 

 magma. It will be observed that such aggregates were met with 

 only in this locality, and I cannot subscribe to the view that the 

 normal garnet of the garnet-gneisses is a product of absorption of 

 aluminous sediment by the magma. This point is dealt with on a 

 later page (see p. 94). 



A point of interest to be further noticed regarding spinel in 

 these aggregates is, that it is never seen in contact with quartz, 

 although it abuts against ])lagioclase. Whether on spinel of 

 hercynitic type becoming enclosed in contact with free silica of the 

 magma, there results the formation of cordierite, or of garnet and 

 sillimanite, has been already discussed in the paper just cited,^ and 

 calls for no further remark here. 



The occurrence of garnet and pyroxene (both diopside and 

 hypersthene), which, in addition to the dominant biotite and horn- 

 blende, are important constituents of the gneissic rocks, calls for 

 some remark. Dealing first with garnet, we may note that Dr. A. 

 Osann-" concludes that its presence in granites indicates an abnormal 

 content of alumina in the magma, and suggests that it is derived 

 by absorption of sediment. To this view, from a study of the 

 Archaean granites of Finland, both Dr. J. J. Sederholm^ and 

 Dr. P. Eskola'^ have subscribed. The application of this ]3rinciple 

 on a general scale to account for the uniform appearance of garnet 

 in great granitic tracts is, however, a doubtful procedure, and the 

 presence of that mineral is probably explicable on other grounds. 

 There seems to be little reason to doubt that garnet can appear 

 as a normal p^a-ogenetic minei'al, and the probability that it 



1 Geol. Mag-. 1921, pp. 251 -59. 

 '^ Ibid. pp. 305 et seqq. 



^ ' Petrochemische Untersucliungen ' Abhandi. Heidelb. Akad. AVisseuscb. 

 No. 2, pt. i(1913)p. 21. 



•* Fennia, vol. viii (1892) No. 3, p. 20. 



5 Bull. Comm. Geol. Finl. No. 40 (1914) pp. 30-31. 



i2 



