68 
SUBTERRANEAN VOLCANIC ACTION 
KOOK I 
series, it might with some confidence be set down as marking the place oi 
one of the active vents from which that series was ejected. 
The chief contrast in external form between this type of neck and that 
formed of fragmentary material arises from differences in the relative dura- 
bility of their component sulistance. The various kinds of lava-form rock 
found in necks are, as a whole, much harder and more indestructible than 
agglomerates and tuffs. Gonsecpieutly bosses of them are apt to stand out 
more prominently. They mount into higher points, present steeper declivi- 
ties, and are scarped into more rugged crags. But essentially they are 
characterized by similar conical outlines, and by rising in the same solitary 
and abrupt way from lower ground around them (see Bigs. 109, 133, and 
195, 294). 
Various joint-structures may be observed in these necks. In some 
cases there is a tendency to separate into joints parallel to the bounding 
walls, and occasionally this arrangement goes so far that the rock has 
acquired a fissile structure as if it were composed of vertical strata. In 
other instances, the rock 
shows a columnar struc- 
ture, the columns diverging 
from the outer margin, or 
curving inwards, or dis- 
playing various irregular 
groupings. More usually, 
however, this jointing is 
so indefinite that no satis- 
, , factory connection can be 
Ft<!. 28. — Section of neck filled with luassive rock. y . 
traced between it and the 
walla of the orifice in which the rock has solidified. 
Some of tlie most remarkable examples of necks ever figured and 
described are those to which attention was called by Captain Dutton as 
displayed in the Zulii plateau of New Mexico, where, amid wide denuded 
sheets of basalt, numerous prominent crags mark the sites of eruptive vents. 
The basalt of these eminences is columnar, the columns standing or lying 
in all sorts of attitudes, and in most cases curved.^ In the Upper Velay, in 
Central France, numerous conspicuous domes and cones of phonolite rise 
amidst the much-worn basalt-plateau of that region (Fig. 345). Many 
instances will be cited in later chapters from the British Isles. 
iii. Distribution of Vents in Relation to Geological Structure-lines 
Where tlie positions of true volcanic necks can be accurately determined, 
it is interesting to study their distribution and their relation to the main 
lines of geological structure around them. Sometimes a distinct linear 
arrangement can be detected in their grouping. Those of the Lower Old 
Bed Sandstone of Central Scotland, for instance, can be followed in lines for 
1 U.S. Gcol. Survey, 6th Annual Rejiorl, 1884-85, p. 172. 
