Prácticas de lectura en bibliotecas virtuales en contextos didácticos diversos

Este trabajo se propone examinar los procesos cognitivos de alumnos de escuelas primaria en las prácticas de lectura en bibliotecas virtuales en el contexto de dos situaciones didácticas contrastantes. La perspectiva teórica adoptada considera los aportes de la tradición socio-histórica vigotskiana y del constructivismo relacional piagetiano. El diseño metodológico se inscribe en un estudio cualitativo de casos en el que se realiza el seguimiento de las prácticas de lectura de los alumnos mediante observaciones de aula donde se registran minuciosamente las acciones de un grupo frente a la pantalla y los intercambios con sus compañeros y el docente. Estos datos se complementan con encuestas realizadas previamente y entrevistas clínicas individuales, administradas a posteriori de la observación. Los resultados evidencian que los estudiantes parten de ideas provenientes de una historia socio-cognitivo-cultural que guían sus representaciones acerca de las bibliotecas virtuales. Estas ideas entran en constante contradicción con las formas organizativas y los contenidos que las bibliotecas les presentan. Las condiciones e intervenciones didácticas tienen un papel constitutivo en el desarrollo de las prácticas y saberes altamente complejos que requieren las búsquedas. La comparación permite avanzar sobre las situaciones que generan una mayor autonomía de los alumnos.

Geochemical history at DSDP Leg 56 Holes

A geological model of subduction postulated by Karig, Ingle, et al. (1975) and Karig and Sharman (1975) proposes that the sedimentary prism at the foot of the landward wall is being actively built as sediment is scraped off the subducting oceanic and plastered onto the base of the wedge, forming an accretionary wedge containing overthrust sedimentary layers or intense sedimentary folding. Because overlying layers must continually be uplifted and compressed to accommodate new matter at the base, the accreting wedge will provide a geochemical record of this process at or near the Japan Trench. Several recent papers have discussed the metalliferous sediments on the active oceanic ridges. The geochemistry of such sediments is now reasonably well known: generally these deposits are considered products of volcanic processes (Boström and Peterson, 1969; Böstrom et al., 1969; Horowitz, 1970, 1974; Cronan et al., 1972; Cronan and Garrett, 1973). The geochemistry of subduction zone sediments, however, is less well known, and the need for studies of these sediments is particularly urgent if such sediments provide a record of the effects of subduction of oceanic plates under continental crust. Because the Japan Trench contains welldeveloped subduction zone deposits, Leg 56 sampling was of utmost importance to the discovery of how they originate.

Geochemistry and mineralogy of selected carbonaceous claystones at DSDP Hole 76-534

Four dominant depositions of carbonaceous claystones are recognized to have occurred during the early Aptian to middle Albian at Site 534. There are correlations of stable isotope ratios with organic carbon content and of clay content with clay mineralogy of the samples. Almost all organic carbon in these sequences has very negative terrestrial isotope ratios, and the clay of that age indicates predominance of aluminous montmorillonite, which is thought to be of terrigenous origin. It is suggested that development of coastal vegetation belts and deltaic outbuilding with consequent outpouring of land-plant detritus and terrigenous elastics into the deep basins probably led to formation of the "black shale" facies.

Concentrations of major-element oxides and minor and trace elements at DSDP Legs 87, 57, 56 and 31

Sediments from Sites 582 (11 samples), 583 (19 samples), 584 (31 samples), 294 (1 sample), 296 (9 samples), 297 (3 samples), 436 (11 samples), and 439 (3 samples) were analyzed by X-ray fluorescence and/or instrumental neutron activation analysis. Ten major elements and 24 minor and trace elements (including 7 rare earth elements) were determined with these methods. Geochemistry varies systematically with both the site location and sediment age. Such variations are explained in terms of changes in sedimentation processes caused by plate motion and changes in ocean currents.