CHAP. XXXIV 
PETROGRAPHY OF THE DYKES 
1 33 
transverse joints may sometimes be 
parallel bars of light and shade (see the double dyke in Fig. 333). Where 
they traverse not only the general mass of a dyke, but also the “ contem- 
poraneous veins ” which cross it, it may be inferred that these veins were 
injected before the final solidification and contraction of the whole dyke. 
An interesting modification of the 
observed, where, as in the case of the 
Palseozoic “ Eock and Spindle,” at St. 
Andrews (Fig. 222), the molten material 
has solidified in a tubular or spherical 
cavity. The joints then radiate in- 
wards from the outer curved surface. 
The most remarkable instance of this 
structure which I have found among 
the Tertiary volcanic plateaux occurs 
on the east side of the island Fuglo, 
the most north-easterly of the group of 
the Faroes. It is cut in section by 
the face of the precipice, where it Fla — Section of cylindrical vein or dyke, 
„„ j i . cutting the bedded lavas, east side of Fuglo, 
appears as a round mass about 40 Faroe islands. 
or 50 feet in diameter piercing the 
plateau-basalts. A selvage of finer material round its outer edge shows the 
effect of rapid chilling, while the joints diverge from the periphery and 
extend in fan-shape towards the centre (Fig. 238). 
One of the most remarkable exhibitions of joint-structure hitherto 
noticed among the Tertiary dykes is that which occurs in the central 
FlG ' Joint-structures in the central vitreous portion of the Eskdale Dyke (B. N. Peach). 
b’ Vit '' e0US Centl ' al baM ’ Sh0Wi ” S thC P0lys0,,al 8rran 6®i®®ut of 
Scores V?). aller portion of tlle sanle wall to show the detailed structure of the ribs (a a) and their vitreous 
' Pr °atthe surfe r ce. 0f tte weathered face of the vail, showing the way in which the hard ribs or sheaths project 
jitieous band of the Eskdale dyke already referred to. The rock is divided 
0 nearly horizontal prisms, each of which consists of an inner more vitreous 
