130 
THE TERTIARY VOLCANOES 
BOOK VIII 
peculiarities which serve in a general way to mark them off from those of 
an outflowing lava. They are usually smaller and more uniform in size 
than in the latter rock. They are also more regularly spherical and less 
frequently elongated in the direction of flow. Moreover, they are not 
usually distributed through the 
whole breadth of a dyke, but tend 
to arrange themselves in lines 
especially towards its centre (Tig. 
236). In these central bands the 
cavities are largest and depart 
farthest from the regular spherical 
form, so that for short spaces they 
may equal in bulk the mass of 
enclosing rock. In some rare in- 
stances, a whole dyke is composed 
of cellular basalt, like one of the 
lava-sheets in the plateaux, as may 
be seen on the north flank of Beinu 
Suardal, Skye. Mr. Harker has 
observed that an amygdaloidal structure is more common among the earlier 
than among the later dykes of that district. 
Besides the common arrangement of fine-grained edges and a more 
coarsely crystalline centre, instances are found where one of the contrasted 
portions of a dyke traverses the other in the form of veins. Of these, I 
think, there are two distinct kinds, probably originating in entirely different 
conditions. In the first place, they may be of coarser grain than the rest 
of the rock ; but such a structure appears to be of extremely rare occurrence. 
I have noticed some examples on the coast of Renfrewshire, where strings 
of a more coarsely crystalline texture traverse the finer-grained body of the 
rock. Veins of this kind are probably of the same nature as the so-called 
“ segregation-veins,” to be afterwards referred to as of frequent occurrence 
among the thicker Tertiary sills. They consist of the same minerals as the 
rest of the rock, but in a different and more developed crystalline arrange- 
ment, and they contain no glassy or devitrified material, except such portions 
of that of the surrounding groundmass as may have been caught between 
their crystalline constituents. 
The second kind of veins, which, though not common, is of much more 
frequent occurrence than the first, is more particularly to be met with among 
the broader dykes, and is distinguished by a remarkable fineness of grain, 
sometimes approaching the texture of felsite or jasper, and occasionally 
taking the form of actual glass. Such veins vary from half an inch or less, 
up to four or five inches in breadth. They run sometimes parallel with the 
walls of the dyke, but often irregularly in all directions, and for the most 
part avoid the marginal portions, though now and then coming up to the 
edge. They never extend beyond the body of the dyke itself into the 
surrounding rock. Though they have obviously been injected after the 
Fig. 236. — Arrangement of lines of amygdales in 
a dyke, Strathmore, Skye. 
