El abuso del derecho y algunas instituciones afines

CAPÍTULO IV. LA TEORÍA DEL ABUSO DEL DERECHO

2. El abuso del derecho y algunas instituciones afines

3. a boundary in mapmaking and cross-sections, normal to the bedding, indicating the areal borders of a particular stratigraphic unit that is not defined by natural features.

4. an impermeable barrier placed within or beneath a dam to block or deter seepage.

cut-off grade, n. the lowest grade of mineralized material, i.e. material of lowest assay value considered as ore in a particular deposit.

cut-off limit, see ASSAYLIMIT. cut-offspur, see MEANDER.

cut-out, n. a mass of siltstone, sandstone, or shale occupying the space of an erosional channel cut into a coal seam.

cwm, see CIRQUE.

cyanobacteria or blue-green algae, n. one-celled organisms without a well-defined nucleus, that live in marine or fresh water. The cells are often arranged in single or clustered filaments. They produce food by photosynthesis and reproduce by fission. The cyanobacteria can live in very inhospitable environments such as hot springs. They are resistant to ultraviolet radiation and can tolerate low oxygen and light levels. They are among the oldest known organisms and examples have been found which are approximately 3400 Ma old, in structures similar to those of modem

STROMATOLITES.

cycle, n. 1. a recurring period of time, especially one in which certain events or phenomena repeat themselves.

2. a sequence of regular occurrences leading to the eventual restoration of the original state of the process, e.g. a CYCLE OF

SEDIMENTATION.

3. a series of events in which, upon completion, the last phase is unlike the initial phase.

cycle of denudation, see CYCLEOFEROSION.

cycle of erosion, n. also called GEOMORPHIC or GEOGRAPHICCYCLE, is a theory of the evolution of landforms. First set forth by W.M. Davis, it assumes progressive development of landforms. The initial uplift that elevates the land is followed by dissection and denudation of the region by streams.

Further erosion reduces it to an old-age surface, or a PENEPLAIN. The cycle could be interrupted at any time by further uplifts and returned to the 'youthful' stage (rejuvenation). The theory also assumes that the history of a landscape proceeds directly from a known stage according to a predetermined framework but it is now realised that this is tectonically unrealistic; for example, it does not take account of a dynamic equilibrium between the crustal elements of the Earth and the processes that act upon them in continuing isostatic readjustments.

cycle of sedimentation or sedimentary cycle, n. a regularly repeated sequence of changes in the conditions within a sedimentary environment that is reflected in a repeated succession of beds. See also CYCLOTHEM, RHYTHMITE.

cyclic evolution, n. the concept that in lineages of many life forms, evolution took a strong and rapid initial course, which was followed by a long phase of slow and moderate change. Overspecialized or ina-daptive forms became extinct during a final brief episode.

cyclosilicate or ring silicate, n. one of the group of SILICATES in which the SiO4 tetrahedra form rings by linking together, each sharing two oxygen atoms. Regardless of the number of such links in a ring (3, 4. or 6), all the rings have a 1:3 ratio of silicon to oxygen. BERYL, Be3Al2Si6O18, is a cyclosilicate.

cyclothem, n. a recurring series of beds deposited during a sedimentary cycle of the type that was common during the Upper CARBONIFEROUS. Such cycles consist of a repeated sequence of MARINELIMESTONES overlain by marine SHALE, lagoonal shale and SiLTSTONE, deltaic SANDSTONE and COAL. The sequence may be formed when a DELTA builds out into a subsiding area, depositing first shales and siltstones then sandstones over the limestones of the open sea. Coal forms from

the remains of the plants that become established on the top of the delta. When the river changes its course, continued subsidence results in the deposition of limestone once more. Alternatively, there may be repeated episodes of subsidence alternating with stable periods when the basin fills with sediment and terrestrial conditions are reestablished. Because most cyclothems are incomplete, a theoretical cyclothem called the ideal cyclothem is used to represent, within specified stratographic intervak and regions, the optimum succession of deposits during a full sedimentary cycle. Both experimental date and theoretical assumptions are used to construct such a cyclothem.

cylindrical fold, n. a fold described geometrically as a surface generated by a line moving through space in a position parallel to itself. The axis in such folds is perpendicular to the plane represented by the girdle, which is the projection of the plane containing all perpendiculars to the bedding.

Compare CONICALFOLD.

Cyprus-type deposit, n. a copper deposit of the type found in the OPHIOLITE complex in Cyprus. Such deposits are believed to form within the oceanic crust at active spreading ridges. Water in sea-floor rocks, heated by magma, reacts with the sea-floor basalt and takes into solution many

minerals present in the rock. As rising hot water is cooled and diluted by cold sea water, sulphides of elements such as copper, nickel, zinc and cadmium are precipitated in rock fractures. See also VOLCANIC-EXHALATIVEDEPOSIT, THERMALSPRING.

cystoid, «. a member of an extinct group of primitive ECHINODERMS. They were sessile animals and were attached to the sea floor either direcdy or by a stalk. The THECA was ovoid and consisted of a number of calcite plates which were often arranged rather irregularly: five AMBULACRA on the upper surface radiated from the mouth to the brachioles (food-gathering arms). Range, Lower Cambrian to Upper Permian. See also BLASTOID.

D

dacite, n. a flow-banded, often dark-coloured IGNEOUS ROCK that is the approximate extrusive equivalent of GRANODIORITE or TONALITE. Its principal minerals are sodic PLAGIOCLASE and QUARTZ, which occur as PHENOCRYSTS in a glassy to microcrystalline GROUNDMASS. MAFIC

phenocrysts (BIOTITE and/or HORNBLENDE and/or PYROXENES) may also be present. Dacite is difficult to distinguish from RHYOLITE without microscopic examination.

Dalradian supergroup, n. a succession of Late PRECAMBRIAN and CAMBRIAN mainly marine SEDIMENTARY ROCKS, mostly metamorphosed, in parts of Scotland and Ireland.

Danian, n. the bottom STAGE of the PALAEOCENE.

darcy, n. a unit expressing the permeability coefficient of a rock, in calculating for instance the flow of water, gas, or oil. The customary unit is the millidarcy (md), equivalent to 0.001 darcy.

Darcy's law, n. a law stating that the flow rate of a fluid through a porous material varies directly with the products of the pressure gradient causing the flow and the permeability of the material. The formula that is derived from Darcy's law assumes laminar flow and negligible inertia. It is used in studies of water, gas, and oil from underground structures.

dark mineral, n. any one of a group of rock-forming minerals showing a dark colour in thin section. Examples are biotite, augite, hornblende, and olivine.

dating methods, n. various methods used to determine when a substance was formed. The particular method chosen depends on the nature of the material and the approximate magnitude of age.

The radiometrk dating technique is based on the decay of radioactive isotopes which have very long HALF-LIVES. The age can be calculated from the measured proportions of parent and daughter isotopes using the AGEEQUATION: t = (1/A) ln (1 + d/p), where t is the age of the sample, A. is the decay constant, (which has a characteristic value for each radioactive isotope), ln is log to the base e, and d/p is the present ratio of daughter to parent isotope. The decay constant is related to the

half-life T (the time taken for half the atoms to decay) thus:

T = 0.693/

λ

The reliability of the calculations depends on how accurately the half-life of the radioactive element is known, whether any of the parent isotope and/or the daughter isotope has been removed from or added to the rock after formation, whether corrections can be made for any daughter isotope present when the rock was formed, and the accuracy of the assumption that the rock was formed during a relatively short time interval in

comparison with its age.

Radiometric methods can be used to determine the age of crystallization of an IGNEOUSROCK from a MAGMA, the time of metamorphic recrystallization (with the formation of a new mineral assemblage), the time of uplift and cooling of fold mountains and the age of SEDIMENTARY ROCKS in which new minerals (AUTHIGENIC minerals) formed during deposition.

The principal radioactive elements used in radiometric dating are listed in fig. 24. Either of the two isotopes of uranium may be used in the uranium-lead method, which is useful for rocks over 100 Ma old. It is less useful for younger rocks because the rate of production of ²⁰⁷Pb is too low. Corrections may have to be made for the amount of non-radiogenic lead incorporated into the mineral at the time of formation. Uranium-lead determinations are usually made on ZIRCON and SPHENE, which are associated with many igneous and METAMORPHIC

ROCKS.

The lead-lead method: because of the differences in half-life of the two uranium isotopes, the ratio ²⁰⁷Pb/²⁰⁶Pb is time-dependent and can be used for age determinations. Corrections for the proportion of radiogenic isotopes incorporated at the time of crystallization can be made by means of an ISOCHRON DIAGRAM, using the relative proportion of non-radiogenic ²⁰⁴Pb.

The potassium-argon method can be used to date rocks ranging in age from 4600 Ma to as recently as 30 000 years. This method, too, is less accurate for younger rocks because of the relatively small amount of daughter product that has formed in them. K-Ar methods are suitable for dating igneous and metamorphic minerals such as HORNBLENDE, BiOTITE, MUSCOVITE and NEPHELiNE, and for whole-rock specimens, particularly of fine-grained rocks - LAVAS, and micaceous metamorphic rocks such as PHYLLITE and SLATE. Some sedimentary rocks containing GLAUCONITE can be dated by this method, since glauconite forms at the time of deposition. Problems arise in the K-Ar method because of the diffusive loss Of⁴⁰Ar at temperatures well below those of metamorphic recrystallization (see BLOCKINGTEMPERATURE).

Information about argon loss can be obtained using the ⁴⁰ArZ³⁹Ar method. Irradiation of the specimen converts a known proportion of ³⁹K to ³⁹Ar.

The specimen is then heated in stages to drive off the argon, and the proportion of ³⁹ArZ⁴⁰Ar at each stage is measured, giving a series of dates. If there had been no loss Of⁴⁰Ar since the time of initial crystallization, the dates calculated should be constant; however if ⁴⁰Ar had been lost, an AGE

SPECTRUM is obtained from which it is possible to determine the time of any heating event as well as the time of initial crystallization.

The rubidium-strontium method is rarely used for rocks younger than about 20 Ma unless they have a high content Of⁸⁷Rb. Minerals such as muscovite, biotite, all die potassium FELDSPARS and glauconite can be dated by this method. Whole-rock specimens of igneous and metamorphic rocks which are rich in micas and potassium feldspar can also be dated. Allowance can be made for the amount of ⁸⁷Sr incorporated at the time of crystallization using an ISOCHRON DIAGRAM plotting the ratios Of⁸⁷SrZ⁸⁶Sr against ⁸⁷RbZ⁸⁶Sr (⁸⁶Sr is non-radiogenic). Dates can be calculated

from die slope of the isochron obtained either from minerals from an individual rock (a mineral isochron) or from several whole rock specimens (a whole-rock isochron). The whole-rock isochron may give a different age from a mineral isochron from die same rock because ⁸⁷Sr can diffuse out of the minerals in which it formed but be retained within the rock (usually in calcium-bearing minerals).

The CARBON-14 (radiocarbon) method is often used to date materials and events that are relatively recent; it is particularly valuable for use on materials less than 50 000 years old, as this time span is too brief for

the more slowly disintegrating elements to produce measurable amounts of daughter products. The radiocarbon method has been widely used in dating archaeological finds, and provides the best dating for die Holocene and Pleistocene. The method is based on the assumption that the isotope ratio of carbon in the cells of living things is identical with that in air because of the balance between photosynthesis and respiration. Although several sources of error are introduced into this mediod, such as the inconstant supply of ¹⁴C, bodi in time and latitude, it has proved to be enormously useful.

Fission track dating is based on the spontaneous HSSION of ²³⁸U, which releases large amounts of energy. The damage this causes to the sur-rounding material can be revealed by etching widi hydrofluoric acid. The half-life of the process is about 8 X 10¹⁵ years, and since the number of fission tracks depends only on the amount of uranium in the sample and the time elapsed since it formed, die age can be calculated. This method is useful for dating relatively recent VOLCANIC material (up to 5 Ma), for example volcanic glasses and minerals which contain uranium such as

APATITE, zircon, sphene and mica.

Varves, which are pairs of thin sedimentary layers deposited widiin a one-year period, have been used to correlate the lake deposits of many thousands of years, back into the Pleistocene and Holocene. Each varve pair consists of a fine winter layer and a coarse summer layer. The ages of deposits are determined by counting the pairs in a sample sediment core. See RHYTHMITES.

Dendrochronology, or tree-ring dating, is a technique of dating and interpreting past events, especially palaeoclimates, based on tree-ring analysis.

A core, extending from bark to centre, is taken from a tree. After the rings are counted and their widths measured, the sequence of rings is correlated with 'deciphered' sequences from oher cores. Tree-ring counts are used to calibrate dates diat have been determined by carbon-14 dating. In so doing, it becomes necessary to modify the assumption of a constant ratio of radioactive to nonradioactive carbon at the time when the organic material to be dated ceased to exchange with the atmosphere.

datum, n. 1. a fixed or assumed point, line, or surface used as a base or reference for the measurement of odier values.

2. that part of a bed of rock on which structure contours are represented.

A datum level is any level surface (usually mean sea level) from which elevations are reckoned. The term datum plane or reference plane applies to some permanendy established horizontal surface that serves as a reference for water depth, ground elevation, and tidal data. In seismology, the datum plane is used to eliminate or minimize local topographic effects to which seismic velocity calculations are referred.

datum plane, see DATUM.

daughter element, n. an element formed from another through radioactive decay, e.g. radium, which is the daughter element of thorium.

dead arc, see REMNANTARC.

dead sea, n. a body of water in which there are no normal aquatic organisms, and from which evaporites are being or have been precipitated. The Dead Sea is the type locality.

death assemblage, see THANATOCOENOSIS.

debouch, v. (physical geography), to emerge from a comparatively narrow valley out upon an open plain or area. A debouchement or debouchure is a mouth or outlet of a river or pass.

debouchement, see DEBOUCH. debouchure, see DEBOUCH.

debris, n. 1. also called rock waste, any surface accumulation of material (rock fragments and soil) detached from rock masses by disintegration.

2. rock and soil material dumped, dropped, or pushed by a glacier, or found on or within it.

3. interplanetary material, including cosmic dust, meteorites, comets and asteroids.

debris avalanche, n. a slide of rock debris in narrow channels or tracks down a steep slope, often initiated by heavy rains. Debris avalanches are common in humid regions.

debris cone n. 1. also called alluvial cone, a steep-sloped alluvial fan, usually consisting of coarse material.

2. a mound of snow or ice on a glacier, topped with a debris layer thick enough to prevent ablation of the material underneath.

debris fall, n. the nearly free fall of weathered mineral and rock material from a vertical or overhanging face. Debris falls are very common along the undercut banks of streams.

debris flow, n. a moving mass of mud, soil, and rock fragments of which more than half the particles are larger than sand size. Such flows range in velocity from less than 1 m/yr to 160 km/hr. Compare MUDFLOW.

debris line, see SWASHMARK.

debris slide, n. the rapid sliding or rolling of predominantly unconsoli-dated earth debris without backward rotation in the movement.

Compare SLUMP. decke, see NAPPE.

declination, n. the horizontal angle between true north and magnetic north, which varies according to geographical location. An instrument that measures magnetic declination is called a declinometer.

declinometer, see DECLINATION.

decollement, n. a detachment structure of strata, associated with over-thrusting. It results in disharmonic folding, i.e. independent patterns of deformation in the rocks above and below the decollement. In attempting to explain the extreme disharmonic folding in the Jura Mountains, many proposed theories have assumed the presence of a decollement.

decomposition, see CHEMICALWEATHERING.

deconvolution, n. a technique designed to restore a wave configuration to the shape it is assumed to have taken prior to filtering. It is applied to seismic reflection and other data for the purpose of improving the visibility and clarity of seismic phenomena.

decrepitation, n. the crackling noise caused by the heating of minerals.

decussate texture, n. a metamorphic texture in which PRISMATIC or other inequidimensional minerals are randomly-orientated. It develops under conditions where STRAIN is negligible and is frequendy observed in contact AUREOLES. See also METAMORPHISM.

dedolomitization, n. a process resulting from contact metamorphism, wherein dolomite is broken down into its two components, CaCO3 and MgCO3. The former crystallizes into a coarse calcite, whereas the latter further breaks down into magnesium oxide and carbon dioxide. The magnesium oxide will usually occur in the rock as brucite, whereas in the presence of silica, magnesium silicates, e.g. FORSTERITE, are formed.