CHAP. V RELATION OF VENTS TO GEOLOGICAL STRUCTURE 
69 
distances of many miles (Map Xo. III). Yet when we try to trace the con- 
nection of such an arrangement with any known great lines of dislocation in 
the terrestrial crust, we can seldom establish it satisfactorily. In the case of 
the Scottish Old lied Sandstone just cited, it is obvious that the vents were 
opened along a broad belt of sub.sidence between the mountains of crystalline 
schist on the north, and those of convoluted Silurian strata on the south, 
either margin of that belt being subsequently, if not then, defined by lines 
of powerful fault. Xo vents have risen along these faults, nor has any 
relation been detected between the sites ol the volcanic foci and dislocations 
in the area of ancient depression. 
Indeed, it may be asserted of the vents of Britain that they are usually 
entirely independent of any faults that traverse at least the upper visible 
part of the earth’s crust. They sometimes rise close to such lines of 
fracture without touching them, but they are equally well developed where 
no fractures are to be found. Yow and then one of them may be observed 
rising along a line of fault, but such a coincidence could hardly fail occa- 
sionally to happen. From the evidence in the British Isles, it is qnite 
certain that if volcanic vents have, as is possible, risen preferably along 
lines of fissure in the terrestrial crust, these lines are seldom those of the 
visible superficial faults, hut must lie much deeper, and are not generally 
prolonged upward to the surface. The frequent recurrence of volcanic 
outbursts at successive geological periods from the same or adjacent vents 
seems to point to the existence of lines or points of weakness deep down 
in the crust, within reach of the internal molten magma, hut far beneath 
the horizon of the stratified formations at the surface, with their more 
superficial disiilacemeuts. 
While sometimes running in lines, old volcanic vents of the Vesuvian 
and Buy types often occur also in scattered groups. Two or three may be 
found together within an area of a few hundred yards. Then may come 
an interval where none, or possibly only a solitary individual, may appear. 
And beyond that space may rise another sporadic group. These features 
are well exhibited by the Carboniferous and Bermian series of Scotland, to 
the account of which the reader is referred. 
A large neck may have a number of smaller ones placed around it, just 
as a modern Vesuvian cone has smaller parasitic cones upon its flanks. An 
instructive example of this arrangement is to he seen at the great vent of 
the Braid Hills belonging to the Lower Old Eed Sandstone and described in 
Chapter xx. Other instances may be cited from the Carboniferous and 
Bermian volcanic series (see Figs. 90, 148, 213). 
Not infrequently the irregularities in the ground-plan of a neck, as 
already remarked, may be accounted for on the supposition that they 
mark the site of more than one vent. Sometimes, indeed, it is possible to 
demonstrate the existence of two or even more vents which have been 
successively opened nearly on the same spot. The first orifice having 
become choked up, another has broken out a little to one side, which in 
turn ceasing to be effective from the same or some other cause, has been 
