428 ON THE ROCKS OF GUERNSEY. 
schist differs in some respect from that of Guernsey, so that without 
the very strongest evidence, from field examination, I hesitate to 
admit the possibility of such an association as this Guernsey speci- 
men would require. We might have here a sedimentary rock of 
ancient date, which was in part an arkose of granitoid materials ; but 
so far as I have been able to study rocks of this kind, their evidence 
is not favourable to this identification. But examination of rocks 
from the Highlnads, and of some specimens from the Cheviots 
sent to me by Mr. Clough, has shown me that the results of local 
crushing are occasionally so singular, and, if I may use the phrase, 
rocks thus maltreated play such tricks that I incline to hold this 
to be the most probable explanation of these Guernsey specimens*. 
EXPLANATION OF PLATE XxX. 
(Geological Sketch-Map of the Island of Guernsey.) 
This map is reduced from Grigg’s (Guerin’s) map. It represents 
the outline at high water. At low tide a large additional area is laid 
bare ; many of the bays are then almost deserted by the sea, and 
Lihou becomes connected with the mainland. Only the most im- 
portant roads are marked, either such as the visitor would naturally 
use, or such as are mentioned in the text. The boundaries can only . 
be regarded as approximations to the truth; in particular, the line 
between the gneiss and the diorites is, in the centre of the island, 
almost entirely conjectural. 
DISCUSSION. 
Mr. Repman mentioned some facts illustrating the great hardness 
and durability of the Guernsey granite. These facts proved that 
the Guernsey granite was the most durable of the Channel-Island 
granites, and very much more so than the Aberdeen granites as 
laid down in the West India Dock tramway over forty years back. 
Prof, SrELEY inquired why the author, in his map of Guernsey, had 
grouped the syenites and diorites together; and what were the fel- 
spars which they contained, or the circumstances which showed their 
relation to each other. He inquired what the author intended to 
imply when the gneiss in the south of the island was termed meta- 
morphic, and the granites and other rocks in the north were termed 
igneous. It seemed to the speaker that, under the conditions of 
contortion in which the metamorphic rocks were admitted to be pro- 
duced, it followed that the deeper-seated mass not only became more 
* The above notes were written in ignorance, for the most part, of the results 
of Mr. Hill’s work in the field. In a few cases I have struck out an expression 
of doubt, where it was removed by stratigraphical evidence ; but I leave these last 
paragraphs as they were written, because | think they afford a fair specimen of 
the “guesses after truth” which can be made with a microscope. He was, I 
believe, not quite satisfied at first as to the nature of the rock of which 45 is a 
specimen; but after a reexamination of the locality, he tells me that he is con- 
vinced that this rock is only a case of peculiar local crushing in the mass, of which 
35 is a more normal specimen. 36 is from a rock which is close by 40. This, 
however, consists mainly of quartz and felspar, and can hardly be a part of the 
same rock. 
