CHAP. XXVII 
SILLS, BOSSES AND DYKES OF THE BUYS 
461 
here given. In I’ig. 166 a dyke of compact black basalt is seen on the 
right hand running up the steep slopes of the agglomerate. Some of these 
dykes are distinctly columnar, the columns diverging from the walls on each 
side. Where the encasing agglomerate has been removed by the weather, 
the side of the dyke presents a reticulated network of prism-ends. A 
narrow basalt-dyke of this character near the top of the Bimi vent is 
represented in Fig. 168. 
But dykes also occur apart from vents and without any apparent 
relation to these. They are sometimes associated with sills and bosses in 
such a manner as to suggest that the whole of these forms of injected 
material belong to one connected series of intrusions. Among the Bathgate 
Hills, for example, from which I have already cited instances of sills and a 
boss, the section represented in Fig. 169 occurs. Tet in this same district 
there is a group of large east and west dykes which cut all the othei rocks 
including the bedded lavas and tufls, and must he ot later date than the 
highest part of the Coal-measures (Fig. 155). 
It is difficult to ascertain the age of the dykes which rise through the 
Carboniferous formations at a distance from any iuterbedded sheets of lava 
and tnff, or from anj' recognizable vent. The south-east and north-west 
dy^kes. increasing in number as they go westward, and attaining a prodigious 
development in the great volcanic area of Antrim and the Inner Hebrides, 
are probably of Tertiary date.’- Others may possibly be Permian, while a 
certain number may reasonably he looked upon as Carboniferous. In petro- 
graphical characters the latter resemble the dolerites and basalts (diabases) 
of the finer-grained sills. 
' These are fully described iu Chapters xxxiv. and xxxv. 
