132 
THE TERTIARY VOLCANOES 
BOOK VIII 
entanglement, however, are of less common occurrence than those already 
referred to, where pieces of some deep-seated rock, such as the gabbros of 
Skye, have been carried up in the ascending magma. Occasionally, where 
the enclosed fragments are oblong, they are arranged with their longer axes 
parallel to the walls of the dyke, showing flow-structure on a large scale. 
Mr. Clough has found some dykes near Dunoon which enclose fragments of 
schist nearly three feet in length. 
One of the most interesting of the megascopic features of the dykes is 
the joints by which they are traversed. These divisional planes are no 
doubt to be regarded as consequences of the contraction of the original 
molten rock during cooling and consolidation between its fissure-walls. 
They are of considerable interest and importance, inasmuch as they furnish 
a ready means of tracing a dyke when it runs through rock of the same 
nature as itself, and also help to throw some light on the stages in the con- 
solidation of the material of the dyke. 
Two distinct systems of joints are recognizable (Fig 237). Though 
sometimes combined in the same 
dyke, they are most conspicu- 
ously displayed when each occurs, 
as it generally does, by itself. 
The first and less frequent system 
of joints (a) has been deter- 
mined by lines of retreat, which 
are parallel to the walls of the 
dyke. The joints are then 
closest together at the margin, 
and may be few or altogether 
absent in the centre. They 
are sometimes so numerous, 
parallel and defined towards the 
borders of the dyke, as to split the rock up into thin flags. Where trans- 
verse joints are also present these flags are divided into irregular tesserw. 
In the second or transverse system of joints (6), which is the more 
usual, the divisional lines pass across the breadth of the dyke, either com- 
pletely from side to side, or from one wall for a longer or shorter distance 
towards the other. Where this series of joints is most completely de- 
veloped the dyke appears to be built up of prisms piled horizontally, or 
nearly so, one above another. These prisms, in rare instances, are as 
regular as the columns of a basalt-sheet (see Fig. 166). Usually, however, 
they have irregularly defined faces, and merge into each other. Where the 
prismatic structure is not displayed, the joints, starting sharply at the wall 
of the dyke, strike inwards in irregular curving lines. It is such transverse 
joints that enable the eye, even from a distance, to distinguish readily the 
course of a dyke up the face of a cliff of basalt-beds, for they belong to 
the dyke itself, are often at right angles to those of the adjacent basalt, and 
by their alternate projecting and re-entering angles seam the dyke with 
a b 
Fig. 237. — Systems of joints in the dykes. 
a, parallel ; b, transverse. 
