236 DR JAMES GEIKIE ON 
TREVELYAN in a section as extending from Norderdahl to Leinum, Its general 
features are well described by Attan, Another and thicker basaltic mass is 
figured by TREVELYAN as occurring near Zellatrae (Selletrod) in Osterée. 
These, so far as I know, are the only intrusive sheets which have been 
observed in the northern islands. Vertical or approximately vertical dykes 
and veins, however, are exceedingly numerous. We saw many in the cliffs, and 
these are indicated upon the map, and there are probably many more in those 
regions which we did not visit. We also came upon fragments of close-grained 
blue basalt in many places upon the hill slopes, which had doubtless been 
derived from dykes and veins exposed to denudation on the steep precipices 
above us. All the dykes seem to consist of the same kind of rock—a hard, 
fine-grained compact blue basalt (of the same composition as the bedded 
basalts), abundantly jointed at right angles to its direction, with several more 
or less continuous joints running parallel to its trend. The cross-joints give 
the rock quite a prismatic structure, the prisms being confined between the 
parallel joints, or between these and the walls of the dyke. Thus in one and 
the same dyke there may be several series of prisms ; but as both the dykes 
themselves and their parallel jointing are very irregular, the prisms are 
irregular also. Each dyke sends out numerous smal] veins which ramify in 
all directions, and are invariably minutely jointed across. The connection 
between these veins and the main dyke is often clearly exposed; but very 
frequently this is not the case, and the small threads and veins then appear 
quite isolated, the connection with the dyke having either been removed by 
denudation, or being still concealed behind the visible surface of dolerite 
through which the veins are squirted. Owing to their highly jointed character, 
these dykes fall rapidly before the action of the weather and the denuding 
agents. Thus in the sea-cliffs they almost invariably give origin to caves. 
Nothing can well be more striking than the appearance presented by many of 
these curious dykes. In the sea-cliffs they generally appear superficially black 
or dark green, and contrast very strongly with the ruddy coloured dolerites 
and tuff across which they break. I give sketches of a few of the more 
remarkable ones I saw (Plates XIV. and XV. figs. 14-17). Fig. 14 represents a 
set of dykes which are probably part and parcel of one and the same intrusive 
mass. All the appearances connected with this and other similar dykes gave 
one the impression that the basalt at the moment of intrusion must have been 
in a condition of extreme fluidity. Nowhere that we saw did the dykes disturb 
the bedding—they seemed to have cut the dolerites very much as a knife cuts 
cheese. Another very remarkable dyke is shown in fig. 16 a, 6, Plate XV. 
The decomposition and erosion of this dyke have given rise to a curious cave 
which forms a kind of natural arcade with a double entrance, as shown upon 
the plan (fig. 16 a). The dyke is in the form of a cross, and the two limbs 
