56 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
eminences known as “Necks” (Fig. 23. See also Figs. 52, 82, 102, 109, 
123, 133, 144, 178, 192, 195, 203,204, 209, 294, 298, 306 and 310). 
Their outlines, however, vary with the nature of their component materials. 
The softer rocks, such as tuffs and agglomerates, are apt to assume the form 
of smooth domes or cones, while the harder and especially the crystalline 
rocks rise into irregular, craggy hills. Occasionally, indeed, it may happen 
that a neck makes no prominence on the surface of the ground, and its 
existence may only be discoverable by a careful examination of the geological 
structure of the locality. Now and then an old vent will he found not to 
Fig. 23. — View of aii old voleanie “ Neek ” (The Knock, Largs, Ayrshire, a vent of 
Lower Carboniferous age). 
form a hill, but to sink into a hollow. Such variations, liowever, have 
little or no reference to original volcanic contours in the history of the 
localities which display them. They arise mainly from the differing hard- 
ness and structure of the materials that have filled the vents, and the con- 
sequent diversity in the amount of resistance which they have offered 
to the progress of denudation. 
Tlie materials now found in volcanic funnels are of two kinds : 
1st, Fragmentary, derived from volcanic explosions; and 2nd, Lava-form, 
arising from tlie ascent and consolidation of molten rock within the funnel. 
i. JVecks of Fragmentary Materials 
By far the most satisfactory evidence of a former volcanic orifice is 
furnished by a neck of fragmentary materials. Where “ bosses ” of crystalline 
rock rise to the surface and assume the outward form of necks, we cannot 
always be certain that they may not have been produced by subterranean 
intrusions that never effected any connection with the surface. In other 
words, such bosses may not mark volcanic orifices at all, though they may 
have been part of tlie underground protrusions of volcanoes in their 
