420 PROF, T. G. BONNEY ON THE MICROSCOPIC STRUCTURE 
lie the islands of Herm and Jethou. The rock of Herm has always 
been considered white granite. That of Jethou has a marked 
structure in vertical planes, but otherwise closely resembles this. 
These vertical planes run north and south, and therefore parallel to the 
most frequent strike of the foliation and folding in Guernsey. Shall 
we not here also find a case of a granite in which structure has been 
developed? Writers have talked of gneiss metamorphosed into 
granite. May not the process in some cases have been the very 
reverse, and some gneisses be mere alterations of granite? Most 
certainly, however, this island of Guernsey must not for the future 
be quoted as affording undoubted instances of syenites metamorphosed 
out of sedimentary rocks. 
I am fully conscious of the deficiencies of this paper. Additional 
study would have removed some, but would certainly have suggested 
many more problems in their turn demanding solution. I have 
thought it best to publish the results already attained, and not to 
delay indefinitely. Besides, work such as would be necessary for 
the complete elucidation of the geology of Guernsey can hardly be 
done except by a resident; and perhaps this paper may be the means 
of inducing some resident to take up this most interesting inquiry. 
APPENDIX. 
Notes on the Microscopic Structure of some Rocks from GUERNSEY. 
By Prof. T. G. Bonnuy, D.Sc., F.R.S., Pres. G.S. 
GNEIss. 
In describing the specimens which Mr. Hill has selected and for- 
warded to me for microscopic examination, it will be convenient to 
group them lithologically rather than geographically. We will 
take first a series illustrating the gneiss, which occupies three fourths 
of the whole area of the island. These slides indicate that this 
region is occupied by a series of coarsely crystalline, rather grani- 
toid gneisses, which have a general lithological resemblance to the 
more typical varieties of the Hebridean group of Scotland. It can- 
not, I think, be doubted that we have exposed here in the Channel 
Islands a fragment of the foundation stones of the earth, of rocks 
roughly corresponding in age with those that on the continent of 
America have been named Laurentian. A lengthy description is 
needless, as so many accounts have now been published of rocks of 
this character ;. enough to say of these, as a whole, that while in- 
dubitably not igneous, they do not generally exhibit a very marked 
foliation. Quartz and felspar are always present, and the third 
predominant mineral is usually a brown mica; the quartz, as seems 
to be commonly the case in these old gneissic rocks, is rather full of 
enclosures, many of which are certainly cavities. These vary much 
in form, some being very irregular, others rounded in outline. 
