HOENBLENDE-SCHISTS, GNEISSES, ETC. OF SARK. 137 



the last stage, and that this produced the slight foliation ; (4) that 

 the rock exhibits an imperfect mixture of two magmas of somewhat 

 diiferent chemical composition, and this mixture may possibly be 

 the result of a quasi-intrusive process on the part of the more 

 f elspathic magma, though very probably neither was it perfectly fluid 

 nor the dark one perfectly solid/ 



We trust then that we have demonstrated from the evidence dis- 

 cussed above that a rock, which macroscopically and microscopically 

 would be recognized as a perfectly normal banded gneiss, can be 

 produced by fluxional movement in a mass, as yet either uncon- 

 solidated or imperfectly consolidated, and consisting of somewhat 

 different materials.^ 



This rock exhibits, as has been pointed out, many distinctive 

 characters. But we find similar characters in a large number of 

 gneisses and crystalline schists. For instance, they are present in 

 the group of the banded gneisses, which, as described above, are so 

 abundant in Sark, and in parts of the hornblende-schist (making 

 allowance for differences which are caused where that mineral pre- 

 dominates). We must not be rash in drawing conclusions, for we 

 have not attempted to examine all the varieties of the gneiss and 

 of the hornblende-schist. We have only studied the types which 

 were most characteristic of the one, and those which promised to 

 be most fruitful of results in the other. Eut each case leads us to 

 the same conclusion. Allowing for the slight effects of subsequent 

 mechanical disturbances, the banded gneiss described in section (c) 

 appears to us structurally inseparable from that of which the origin 

 has been, in our opinion, demonstrated. Again, the broad-banded 

 variety of hornblende-schist described in (b) is produced by the 

 interstreaking of a granitic and dioritic rock, and so is really more 

 closely related to one of the normal members of the Lizard Granu- 

 litic Group than to one of the Hornblendic Group. Yet from this 

 variety, which must be similarly explained, it does not seem possible 

 to separate the normal hornblende-schist of the island, in which the 

 materials appear to be more completely mixed, so that a slight 

 foliation, but not a banding, has been produced by the fluxional 

 movement. 



Thus our investigations, so far as they go, are in favour of 

 assigning an igneous origin to these banded rocks (gneisses and 



^ We do not suppose that when the intrusion occurred the rocks were exactly 

 in their present position, but that they have moved onwards together. 



^ After this paper had been forwarded to the Society my attention was called 

 in conversation to a section in Rejer's ' Theoretische Geologic,' p. 810, where he 

 makes some valuable remarks on the occurrence of similar structures (Schliere). 



[Reference should also be made to the cases quoted by Mr. Lawson in his 

 paper printed in the ' Comptes Rendus, Congres Internat. Geol.' (1888) p. 143, 

 and described at greater length in his memoir on the ' Geology of the Rainy 

 Lake Region ' ^ but the possibility of such a process had been present for some 

 time previously in my own mind, and I may say that the conclusions in this 

 paper have been arrived at independently. Indeed, the absence of an important 

 link in the chain of reasoning in the first paper caused me to pay less attention 

 to it than the second showed it to deserve. Be it understood that I make no 

 claim to priority.] — T. G. B. 



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