strata. Many volcanic necks stand on lines of fault. In other cases
there are groups of necks lying in a straight or sinuous line, which
may indicate the position of a fracture or at least of a line of least
resistance. But in Scotland it is often impossible to adduce any
evidence of the connexion between faults or fissures and the position
of volcanic necks; and it seems likely that the pressure of the gases
in the igneous magma increased till an explosion took place which
perforated the rocks above with a clean tubular passage often nearly
circular in cross section. This pipe was usually vertical, and nearly
uniform in diameter for great depths; the material occupying it,
when exposed by denudation, has a circular ground plan, or if
shown in vertical section (or elevation) in a cliff is a pillar-shaped
mass crossing the bedding planes of the strata nearly at right angles.
It terminates upwards in the remains of the volcanic cone and communicates
below with the reservoir from which the lavas were
emitted, represented in most cases, where it has been exposed, by a
large irregular mass (a batholith or boss) of coarsely crystalline
igneous rock. The site of such a neck is generally indicated by a
low conical hill consisting of volcanic rock, surrounded by sedimentary
or igneous strata of a different kind. The low cone is due
to the greater hardness and strength of the volcanic materials and
is not connected with the original shape of the volcano. Such hills
are common in some parts of Scotland and well-known examples are
Arthur’s Seat and the Castle Rock (Edinburgh), North Berwick Law,
the Bass Rock; they occur also in the Peak district of Derbyshire,
and the Wolf Rock off the coast of Cornwall is probably a neck. Two
splendid sugar-loaf cones known as the Pitons of St Lucia in the West
Indies, rising from the sea with almost vertical sides to a height of
nearly 3000 ft., are old volcanic necks. In Texas, New Mexico,
Arizona, California and many of the western states of North
America geologists have observed conical volcanic hills having all
the features which belong to necks.
Where the volcanic rocks are soft and easily disintegrated they may be reduced more rapidly than the strata around them and the position of a neck may be indicated by a cup-shaped hollow; this is the case with some of the diamond-bearing basic pipes of South Africa. Sometimes necks are encountered in underground mining operations; in the coal-field of Fife, for instance, the coals are sometimes replaced by a circular mass of volcanic rock, a quarter of a mile or more in diameter, which rise vertically to the surface. Better examples are the Kimberley diamond mines. The blue-ground (or serpentine breccia) occupies great pipes or funnels, circular in outline with nearly vertical sides, extending downwards to unknown depths; these are undoubtedly the necks of old volcanoes. If any lavas were poured out from these pipes at the surface they have since been carried away by denudation.
The size of necks varies considerably; the smallest may be only 20 or 30 yds. in diameter, the largest are several miles. In this respect they resemble active craters, but no necks have been met with on the earth’s surface with dimensions approaching those of the so-called “craters” of the moon. Small necks are usually simple, i.e. they contain only one or two kinds of igneous rock (ashes and dikes) and have been produced, so far as we can judge, by a single eruption. Not infrequently they contain no volcanic rock; but are filled with pieces of slate, sandstone or whatever strata the pipe traverses. Such necks must have been produced by a single eruption with an outburst of steam, not followed by lava; the disrupted fragments of the surrounding rocks and the materials tumbling down from the crater’s walls ultimately filled up the cavity. Instances occur in Fifeshire and in Shetland and among the recent volcanoes of the Eiffel there are some which have thrown out more slate and sandstone than lava.
Large necks, on the other hand, are often of complex structure, contain many kinds of rock and seem to have been produced by repeated eruptions, each of which more or less completely cleared out the material obstructing the orifice, and introduced a series of fresh accumulations. The beds of ashes which line the interior of an active crater have in nearly all cases a slope or dip towards a central point where the base of the depression is situated, and in volcanic necks which have been filled with ash (tuffs and agglomerates) this funnel-like inward dip is very constant. If there has been only a single eruption the beds of ashes have a very conformable or uniform arrangement, but if activity has been resumed after a period of quiescence a large part of the old material may have been projected and a new series of beds laid down, transgressing unconformably the edges of the earlier ones. By these structures we can sometimes trace a neck within a neck, or of a lateral crater on the margin of a principal one.
Where the crater has filled up with very coarse ashes, or agglomerate, the bedding is rarely visible. Sometimes large empty craters were occupied temporarily by lakes, and level sheets of mud and silt have gathered on their floors: hence bedded sediments are not infrequently found in volcanic necks. Mixed with the volcanic ashes and bombs there are often large broken pieces of sedimentary rocks which may have been crystallized and hardened by the heat and vapours emitted by the volcano. Sometimes great fragments of the walls have foundered or collapsed into the crater, and masses of non-volcanic rock, an acre or more in extent, may occur in a volcanic neck. In Arran, for example, there is a large neck which contains lumps of Cretaceous rocks nowhere else known to occur on the island; they have fallen down from strata once occupying part of the walls of the crater but now removed by denudation.
The lava which rises and flows out from the crater leaves its trace also in the necks. Sometimes it forms thin beds or flows alternating with the tuffs and having the same basin-shaped dip. More commonly it appears as the material filling fissures and pipes, traversing the ashes irregularly or rising as a central plug in the interior of the neck, and sending out branching veins. Occasionally a whole neck is composed of solid crystalline rock representing the last part of the magma which ascended from the underground focus and congealed within the crater. In Mont Pelée, for instance, the last stage of the eruptions of 1902 to 1905 was the protrusion of a great column of solidified lava which rose at one time to a height of 900 ft. above the lip of the crater, but has since crumbled down. The Castle Rock of Edinburgh is a neck occupied by a plug of crystalline basalt. Necks of this kind weather down very slowly and tend to form prominent hills.
After the eruptions terminate gases or hot solutions given out by deep-lying masses of molten rock may find a passage upward through the materials occupying the crater, greatly modifying their mineral nature and laying down fresh deposits. A good example of secondary deposits within a volcanic neck is provided by the Cripple Creek mining district of Colorado. The ore-bearing veins are connected with volcanic rocks and part of these occupy a vertical circular pipe which is a typical volcanic neck. A phonolitic breccia, greatly altered, is the principal rock, and is cut by dikes of phonolite, dolerite, &c. The country rock is mostly granite and gneiss, and blocks of these are common in the breccia. A large volcano was built up in Tertiary times on the granite plateau, and has since been almost entirely removed by denudation. The gold ores were carried upwards by currents of hot water derived from the volcanic magma and were deposited along cracks and fissures in the materials which occupied the crater, and also in the surrounding rocks (see Volcano). (J. S. F.)
NECKAM, ALEXANDER (1157–1217), English school man
and man of science, was born at St Albans in September 1157,
on the same night as King Richard I. Neckam’s mother nursed
the prince with her own son, who thus became Richard’s foster-brother.
He was educated at St Albans Abbey school, and began
to teach as schoolmaster of Dunstable, dependent on St Albans
Abbey. Later he resided several years in Paris, where by 1180
he had become a distinguished lecturer of the university. By
1186 he was back in England, where he again held the place
of schoolmaster at Dunstable. He is said to have visited Italy
with the bishop of Worcester, but this statement has been
doubted; the assertion that he was ever prior of St Nicolas,
Exeter, seems a mistake: on the other hand, he was certainly
much at court during some part of his life. Having become
an Augustinian canon, he was appointed abbot of Cirencester
in 1213. He died at Kempsey in Worcestershire in 1217, and
was buried at Worcester. Besides theology he was interested
in the study of grammar and natural history, but his name is
chiefly associated with nautical science. For in his De naturis
rerum and De utensilibus (the former of which, at any
rate, had become well known at the end of the 12th century, and
was probably written about 1180) Neckam has preserved to
us the earliest European notices of the magnet as a guide to
seamen—outside China, indeed, these seem to be the earliest
notices of this mystery of nature that have survived in any
country or civilization. It was probably in Paris, the chief
intellectual centre of his time, that Neckam heard how a ship,
among its other stores, must have a needle placed above a magnet
(the De utensilibus assumes a needle mounted on a pivot),
which needle would revolve until its point looked north, and
thus guide sailors in murky weather or on starless nights. It
is noteworthy that Neckam has no air of imparting a startling
novelty: he merely records what had apparently become the
regular practice of at least many seamen of the Catholic world.
See Thomas Wright’s edition of Neckam’s De naturis rerum and De laudibus divinae sapientiae in the Rolls Series (1863), and of the De utensilibus in his Volume of Vocabularies. Neckam also wrote Corrogationes Promethei, a scriptural commentary prefaced by a treatise on grammatical criticism; a translation of Aesop into Latin elegiacs (six fables from this version, as given in a Paris MS., are printed in Robert’s Fables inédites); commentaries, still unprinted, on portions of Aristotle, Martianus Capella and Ovid’s Metamorphoses, and other works. Of all these the De nat. rer., a sort of manual of the scientific knowledge of the 12th century, is much the most important: the magnet passage herein is in book ii. chap. xcviii. (De vi attractiva), p. 183 of Wright’s edition. The corresponding section in the De utensil, is on p. 114 of the Vol. of Vocabs.
