50 REPORT—1849. 
one must have experienced in attempting to investigate the relations of the mineral 
masses of the Cotentin, arises from the want of natural sections, owing partly to the 
manner in which its surface is covered by heath and wood, and partly to superficial 
accumulations; a difficulty, which, though it exists to considerable extent in the 
larger islands of Guernsey and Jersey, is obviated hy the great extent of coast-line 
they present. 
The sedimentary rocks of Guernsey and Jersey are of inconsiderable extent; a 
small patch of clay-slate occurs in Rocquaine Bay, on the west of Guernsey, and 
larger areas are occupied by it in the north and north-east parts of Jersey: in the 
latter they are occasionally siliceous, and pass into subordinate beds of rounded con- 
glomerate. The whole of this group has been variously moved about, but its general 
slope as a mass is to east. These beds are evidently a part of the palzozoic series of 
the Cotentin, and closely resemble that portion of it which, consisting of alternations 
of compact sandstones and argillaceous shales, are well seen on the north and south 
of Valognes. The calcareous bands of the French series are altogether wanting, nor 
did I see any of those peculiar steaschist beds, with nodules of quartz, which in the 
neighbourhood of Cherbourg underlie the middle part of the group. 
Organic remains, if not entirely wanting, must be exceedingly scarce in these beds, 
in which respect they agree with that part of the French series to which I have com- 
pared them. 
No trace of any one of the secondary series of formations is to be found over these 
islands; the surface of the slate rocks has been much denuded, and the like process 
may have removed whatever newer strata may have at some time existed there ; but 
from certain characters which the new red sandstone and the cretaceous beds put on in 
their extension into the west of France, it is more probable that beds of that period 
never were deposited here. 
The geological interest which attaches to these islands consists in the relative ages of 
the crystalline rocks, which form so large a portion of their masses. A circumstance 
which cannot fail to strike any observer is the very great changes of mineral character 
which these masses put on, and within very narrow limits : there is, however, a three- 
fold division, which is apparent enough :— ; 
1. A flat-bedded crystalline group, such as that which occurs over the southern 
half of Guernsey. 
2. A granitic group, which includes a series of gray, red, and black granites. 
3. A sienitic series, comprising a vast variety of combinations, in which, however, 
hornblende prevails. 
The first of these groups, as it is seen in Guernsey, is of great thickness, and though 
no topographical limits can well be drawn, it may be said to occupy all south of a line 
from Castle Cornet to Vason Bay ; it is in some places a true granite, at others gneiss, 
at others a true micaceous schist. It agrees with the next group as to the constituent 
minerals into which it graduates downwards. 
The true granites are to be seen over the northern part of Guernsey, nowhere better 
than in the quarries of St. Sampson’s parish. These latter, as a mass, underlie the 
first group, and their more massive external character, as well as more uniform cry= 
stalline texture, may be merely the results of cooling under rather different condi- 
tions; and the whole may be an intrusive plutonic mass of the same period. On the 
other hand, portions of the upper group irresistibly suggest the notion that at some 
time they must have existed as sedimentary strata. 
In the island of Jersey the true granites occupy the southern portion, and it is. only 
here that we see their relation to the slate series already noticed. As the slates ap- 
proach the granite, they become hard and splintery. At this junction enormous veins 
or branches extend from the granitic mass, as well as the most delicate threads; at 
places the slate rocks seem reduced to fragments, among which the fluid granite has 
poured itself, the angular edges being sharp and uninjured. 
The granite of this island puts on a character more closely resembling stratification 
than is usually seen; so much so, that in many places,’as for instance in the steep 
walls of rock beneath the citadel of St. Heliers, it might be, excusably mistaken for a 
mass of highly inclined sedimentary beds. These division& haye no general angle of 
dip, but are inclined most unequally ; they may be planes Of cooling which were once 
horizontal, and have acquired their present position by subsequent disturbance. The 
extreme smoothness of their surfaces is very remarkable. 

