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Encyclopedia Britannica - Main :: NAN-NEW |
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NECKAM, ALEXANDER (1157-1217) , English schoolman 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
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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
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 ; theyand 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 118o 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 118o) Neckam has preserved to us the earliest European notices of the magnet as a guide to seamenoutside 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
See Thomas Wright
manual
Wright
Roger Bacon's reference to Neckam as a grammatical writer (in multis vera et utilia scripsit: sed .. . inter auctores non potest . . nunzerari) may be found in Brewer's (Rolls Series) edition of Bacon's Opera inedita, p. 457. See also Thomas Wright, Biographia Britannica literaria, Anglo-Norman Period, pp. 449459 (1846: some points in this are modified in the 1863 edition of De nat. rer.); C. Raymond Beazley, Dawn of Modern Geography, iii. 508-509. (C. R. B.) End of Article: NECKAM, ALEXANDER (1157-1217) If you wish, you can link directly to this article.
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