i6 
VOLCANIC PRODUCTS 
BOCK I 
not always by itself sufficient to distinguish them from the intrusive rocks. 
Examples will be given in later chapters where dykes, sills and other 
masses of injected igneous material are conspicuously cellular in some parts. 
F)Ut, in such cases, the cavities are generally comparatively small, usually 
spherical or approximately so, tolerably uniform in size and distribution, 
and, especially when they occur in dykes, distributed more particularly 
along certain lines or bands, sometimes with considerable regularity (see 
Pigs. 90, 91, and 236). 
Among the superficial lavas, however, such regularity is rarely to be 
seen. Now and then, indeed, a lava, which is not on the whole cellular, 
may be found to have rows of vesicles arranged parallel to its under or 
upper surface, or it may have accpiired a peculiar banded structure from the 
arrangement of its vesicles in parallel layers along the direction of flow. The 
last-named peculiarity is widely distributed among the Tertiary lavas of 
North-Western Europe, and gives to their weathered surfaces a deceptive 
resemblance to tuffs or other stratified rocks (see Figs. 260, 310 and 311). 
It will be more particularly referred to a few pages further on. In general, 
however, we may say that the steam-cavities of lavas are quite irregular in 
size, shape and distribution, sometimes increasing to such relative propor- 
tions as to occupy most of the bulk of the rock, and in other places dis- 
appearing, so as to leave the lava tolerably compact. When a lava presents 
an irregularly vesicular character, lilce that of the slags of an iron-furnace, it 
is said to be datfjy. AVhen its upper surface is rugged and full of steam- 
vesicles of all sizes up to large cavernous spaces, it is said to be ^coriaceous, 
and fragments of such a rock ejected from a volcanic vent are spoken of as 
scoriae. 
xittention to the llattening of the steam-vesicles in cellular lavas, which 
has just been alluded to as the result of the onward movement of the still 
molten mass, may show, by the trend and grouping of these elongated cavities, 
the probable direction of the flow of the lava before it came to rest. Some- 
times the vesicles have been drawn out and flattened to such a degree 
that the rock has acquired in consequence a fissile structure. In other 
instances, the vesicles have been originally formed as long parallel and even 
branching tubes, like the burrows of Annelids or the borings of Teredo. 
Some remarkable examples of tliis exceptional structure have been obtained 
from the Tertiary plateau-basalts of the Western Isles, of which an example 
is represented in Eig. 2. 
In many cases the vesicles extend through the whole thickness of a 
lava. Frequently they may be found most developed towards the top and 
bottom; the central portion of the sheet being compact, while the top and 
bottom are rugged, cavernous or scoriaceous. 
Though originally the vesicles and cavernous spaces, blown open by the 
expansion of the vapoixrs dissolved in molten lava, remained empty on the 
consolidation of the rock, they have generally been subsequently filled up 
by the deposit withirr them of mineral substances carried in aqueous solu- 
tion. The minerals thus introduced are such as might have been derived 
