40 



DR GEIKIE ON THE HISTORY OE VOLCANIC ACTION 



they are arranged with their longer axes parallel to the walls of the dyke, showing 

 flow-structure on a large scale. Mr Clough has lately found some dykes near Dunoon 

 which enclose fragments of schist nearly three feet in length. 



One of the most interesting of the macroscropic features of the dykes, is the joints by 

 which they are traversed. These divisional planes are no doubt to be regarded as con- 

 sequences 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 consolidation of the 

 material of the dyke. 



Two distinct systems of joints are recognisable (fig. 3). Though sometimes combined 

 in the same dyke, they are most conspicuously 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 transverse joints 

 are also present, these flags are divided into 



a b 



Fig. 3. — Systems of Joints in the Basic Dykes. 

 a, parallel ; b, transverse. 



irregular tesserce. 



In the second or transverse system of joints (b), which is the more usual, the divisional 

 lines pass across the breadth of the dyke, either completely from side to side or from one 

 \v T all for a longer or shorter distance towards the other. Where this series of joints is 

 most completely developed 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. 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 

 are banded across with parallel bars of light and shade. Where they traverse not only 

 the general mass of a dyke, but also the " contemporaneous veins" which cross it, it may 

 be plausibly inferred that these veins were injected before the final solidification and 

 contraction of the whole dyke. 



One of the most remarkable exhibitions of joint-structure hitherto noticed among 

 these dykes, is that which occurs in the central vitreous band of the Eskdale dyke 

 already referred to. The rock is divided into nearly horizontal prisms, each of which 



