REV. E. HILL ON THE ROCKS OF GUERNSEY. 417 
on Les T'albots Road was given the name of “ Talvan” by the manager 
of the Bouet quarry, in conversation withme. ‘Tothis group belong 
some dykes with pink blotches sometimes half an inch across, of 
which the best example occurs at the west point of Perrelle Bay; 
while I have other specimens from Leree, Jerbourg Point, and 
Castle Cornet. The pink blotches appear to be nodes of felspar 
surrounding a grain of quartz. The group, as a whole, appears to 
have been also originally diorite. The commonest type of rock 
among the dykes is compact crypto-crystalline and highly jointed, 
dark green or grey and slate-like. These dykes cut in sharply 
defined walls across all the principal rock-masses alike. They are 
perhaps diabases. The great intrusions, often 30 feet thick, which 
seam the southern cliffs are a characteristic feature of the scenery. 
The whole cliff of Pleinmont Point is formed by a mass of this kind, 
fully half a mile in length. Another group of large intrusions is 
conspicuous below high-water mark round the peninsula between 
Vazon and Perelle Bays. In the quarry south of Fermain Bay, and 
at other places, dykes occur in which a slaty cleavage has been 
developed. Along the cliffs of the south are various included 
schistose-looking masses which may in some cases be contorted 
beds, but in most cases, I think, are intrusive, as for instance in 
the cliffs about Torteval. On the shore west of Mont Crevelt a 
vertical band of rock is visible for a hundred yards or more, which 
can scarcely be distinguished in any way from a slate. But it is 
marked sharply off from the rock which it traverses ; there is not 
the vestige of a transition, or of any bedding corresponding to it; 
and the boundaries are sinuous, so that the breadth varies irregularly. 
I am confident that this also is a dyke. 
Dykes decomposed into an earthy state occur at Hougue a la Perre 
and Hommet Bennest. 
Certain black stripes which traverse the ‘ Birdseye’ (gabbro) at _ 
Hougue 4 la Perre and elsewhere are also no doubt intrusions. At 
the east entrance of the Delancy quarry can be seen an excellent 
instance. As it sends out strings and has a compact selvage, no one 
will question that it is an intruder. The microscope shows that it 
is hornblendic. 
A singular type of rock is afforded by the great intrusion at Bon 
Repos Bay. This covers the whole floor of the bay, and extends 
half way up the cliff and apparently across Corbiere Point. It 
consists of a closely felted mass of hornblende crystals, about + inch 
long, often with no other visible essential constituent, though in parts 
there is some white felspar. J could not actually reach a junction, 
but have no doubt of its being intrusive in the gneiss, and I found 
a piece of what seemed to be gneiss imbedded init. No doubt tobe 
grouped with this, is a dyke at Leree, near high-water mark just 
north of the Lihou causeway, a mass of hornblende crystals of 
remarkably perfect external form. A rock occurring in the quarries 
of ‘‘ Birdseye” and called by the workmen there “ Long-grain” appears 
to be another variety. It has a platy cleavage which no doubt gives 
rise to the name. From the men’s description I think it must occur 
ee ~~. 
