The metamorphosed igneous rocks of Moulin Huet Bay in the Channel Island of Guernsey, the Icart Gneiss, are traversed by later intrusions of molten volcanic rock that filled spaces where fractures opened up in the gneiss. These intrusive rocks are sheet-like formations of varying extent and thickness, appearing on the weathering rock surface as narrow bands of contrasting colour and texture, and they are known as dykes. Dykes are igneous rocks that can be composed of different combinations of minerals, and they can also be metamorphosed later into yet more compositions. On Guernsey there are apparently six different types of basic (as opposed to acid) dyke and it is difficult to distinguish between these types when just observing in the field. According to the simple guide written by Pomerai and Robinson (1994) most of the dykes at Moulin Huet are made of dolerite, as shown in the examples illustrating this post. [There are also a couple of lamprophyre dykes which I will show in a separate post]. I am aware that this identification as dolerite may be an over simplification but will investigate further.
The photographs here show the contrasting textures and colours of the rocks, with the relatively fine-grained, smooth, and homogenous grey-green dolerite dykes within the coarse-grained Icart Gneiss and its large, squashed pink-orange feldspar crystals. In some instances, there are pieces of the Icart Gneiss within the dolerite, these having broken off the sides of the bedrock and become incorporated into the molten lava as the dyke was formed – these inclusions are called xenoliths.
De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8.