196 STRUCTURAL AND FIELD GEOLOGY 
3. NECKS 
Necks are pipes or conduits of eruption—the throats, in 
short, of old volcanoes. They are filled either with crystalline 
rock or fragmental materials, or with both. They are of less 
deep-seated origin than batholiths ; indeed, portions of the old 
volcanic cone are still to be seen surrounding a neck in some 
cases. As a rule, however, the cones have been entirely 
demolished—only the plugged-up vents remaining. Nota few 
of these seem to represent very small volcanoes—the products 
of single eruptions, like that which, in 1538, gave birth to the 
tuff and cinder cone of Monte Nuovo (Bay of Baiz). Others, 
again, are obviously the relics of much more important 
volcanoes, from which were discharged not only fragmental 
materials but streams of lava. Between necks of this kind 
and certain bosses no hard-and-fast line can be drawn. Some 
of the latter,as we have seen, appear to have had communica- 
tion with the surface, and these, therefore, might equally well be 
described as necks. That term, however, ought rather to be 
reserved for the less important pipes or funnels of eruption— 
most of which, indeed, represent only the uppermost or 
terminal portions of such pipes. For, even in the case of the 
most highly denuded neck, we have no reason to suppose that 
the portion remaining occurred at any great depth below the 
base of the old volcanic cone to which it led. It is conceivable 
that, could we trace an important neck downwards, we should 
find it gradually assume the character of a more or less 
funnel-shaped boss, and this in its turn might, at a greater 
depth still, expand into a yet more extensive batholith. It 
would seem, therefore, as if the structure now presented by 
many an old focus of eruption, may have been determined by 
the degree of denudation which it has experienced. With a 
minimum amount of erosion we have the cone of the extinct 
volcano, still recognisable as such. Increased erosion removes 
the cone, and then only a neck remains; until after some 
prolonged period the whole region becomes so reduced that 
the batholith or more deeply-seated portion is laid bare. 
Seen in groundplan, typical necks tend to be more or less 
circular or elliptical in form, but they are frequently irregular. 
Occasionally, however, such irregular shapes are suggestive 

