CHAP. XVIII 
SILLS AND DYKES 
293 
have been mapped, which must be regarded as due to the same series 
of movements and protrusions of the magma that produced the adjacent 
sills. Throughout the length of the Southern Uplands dykes of felsite, 
rninette, lamprophyre, vogesite and other varieties, which may also be 
connected with the volcanic phenomena of the Lower Old Led Sand- 
stone, not infrequently occur among the Silurian rocks. On the 
Kincardineshire coast, south of Bervie, a number of dykes of pink 
quartz -porphyry traverse the conglomerates and sandstones. The coast 
south of Montrose displays some singularly picturesque sections, where a 
porphyry dyke running through andesitic lavas and agglomerates stands up 
in wall-like and tower-like projections. On the shore at Gourdon, as well 
as inland, intrusive dykes of serpentine occur. A line of these, possibly 
along the same fissure, lias been traced for more than a dozen of miles fioni 
above Cortachy Castle to near Bamff. But there is no evidence to connect 
them with the volcanic phenomena of the Old Bed Sandstone. Not improb- 
ably they belong to a later geological period. 
One would expect to meet with a network of dykes in and around the 
volcanic vents ; lint even there they are usually not conspicuous either for 
number or size. In the great vent of the Braid Hills only a few have been 
noticed. In the Ochil Hills groups of dykes of felsite and andesite may 
be observed, especially near the necks. I'hey are fairly numerous in the 
neighbourhood of Dollar (see Fig. 08). One of the most abundant series 
yet'’ oliserved traverses the tract around the granite boss of the Cheviot 
Hills, from which many dykes of granite, felsite, quartz-porphyry and ande- 
site radiate. This district will be more fully referred to in Chapter xxi. 
Another remarkable development of dykes occurs in Shetland (lig. Z-^), 
where they consist of granite, felsite and rhyolite, and are associated with 
the acid sills above referred to. 
