134 



EEV. E. HILL AND PEOF. T. G. BONNET ON THE 



Banded gneiss. 



du IVToiilin, the banded gneiss is distinctly cut by veins (subsequent 

 to the date of its structure) which macroscopically resemble closely 



the basement-gneiss. More 



Fig. 6. — Granite vein, slightly foli- than one instance of this may 



ated, intrusive in handed gneiss, be seen in Pegane Bay.^ 



^^W^ 1^6gdne Bay. Here is a very rough diagram 



(for to represent the original 

 accurately would require 

 either hours of work with 

 the pencil, or a photograph). 

 The rock of the vein also 

 exhibits in places a slight 

 foliation, so that in a hand- 

 specimen it would be regarded 

 as a gneiss. On microscopic 

 examination we find nothing 

 to separate it from the base- 

 ment-gneiss, except perhaps 

 that it contains rather less 

 biotite. Among the felspar 

 is some microcline. Perhaps, 

 however, the most important 

 fact to which this vein testifies is that here, in an undoubtedly 

 intrusive rock, we find precisely the same structures, viz. the 

 irregular forms of the quartz and felspar, the inclusion of the 

 one by the other, and their occasional micrographic association, 

 as those that are characteristic of the gneissic rocks which have 

 been so often discussed.^ 



Thus, taking account of all the cases observed, our interpretation 

 of the history of these and other banded gneisses seems justified, 

 and on that assumption we think it most probable that the basement- 

 gneiss, instead of being, as it has hitherto been supposed, an older 

 rock than the hornblende-schists and banded gneiss, is really newer, 

 though the difference in age need not be great. 



Here it may be well to notice some cases in Guernsey which 

 corroborate our explanation of the banded structure in these crys- 

 talline rocks,^ viz. that it is due to a fluxional movement anterior to 



' Pegane Bay is immediately south of Port du Moulin, and is reached thence 

 by passing through a cleft and round the next promontory, over rocks laid 

 bare by the falling tide. 



^ A parallel case occurs in a headland on the north side of the west bay below 

 the Ooupee. Here we find a diorite intrusive into the gneissic rock. In 

 one part it is a mottled pink and green rock, some of the felspars being rather 

 longer and redder than the others ; in another part a few yards away the red 

 grains remain, but appear in a rather streaky groundmass consisting of the 

 other felspar and of hornblende, recalling some of the streaky 'augengabbros' 

 (though less definite than they) of the Lizard. The one Aariety shades imper- 

 ceptibly into the other, and there is nothing to suggest a crush, for though the 

 gneiss may have suffered from one, that clearly was anterior to the date of the 

 intrusion. 



^ It must not be forgotten that sometimes a slight foliation is exhibited by 

 yeins of dioritic or granitic rock, intrusive (as mentioned below) in the horn- 

 blendic and gneissic group, wJiere there is no sign whatever of any subsequent 

 disturbance, and also in the granitic masses at the N. and S. ends of Sark. 



