Northern Chile, which includes the extremely arid Atacama Desert and the semiarid Andean Highlands, has more than 100 basins with interior drainage; most contain salars (salt-encrusted playas). The area of interior drainage totals more than 38,000 square miles, within which salars and clay playas extend over a total area of about 2,800 square miles. In addition, hills and valleys in the Atacama Desert are extensively covered either with a thin hard saline crust, chiefly salt-cemented soil, or with a powdery soil that has a high content of saline material, chiefly anhydrite and gypsum.The region has an exceptional variety of types of hard saline crusts that are generally rare in other deserts, and many morphological and structural salt features, some of which may be unique. Soft saline crusts and clay playas, more characteristic of arid regions elsewhere, are also present. Hard salar crusts have formed by deposition of saline material in open water or by capillary migration and evaporation of near-surface ground water. Such crusts generally range from a few inches to several feet in thickness. Locally, crusts may attain thicknesses of several tens of feet, and one salar, Salar Grande, is a basin filled with highpurity rock salt to a local depth of at least 560 feet.Six general types of hard salar crusts are distinguished: (1) layered massive rock salt with a rugged surface, (2) slabby or nodular silty rock salt, (3) rugged gypsum or anhydrite, (4) massive coarsely crystalline rock salt, (5) smooth rock salt, and (6) silty nitrate-bearing saline crust. Soft surfaces or crusts include moist gypsum-bearing crusts, which commonly contain nodules and layers of ulexite in Andean salars, and moist to dry puffy soils and crusts that contain gypsum, thenardite and mirabilite as the principal saline constituents. An unusual chemical feature of the salars and the desert soils of northern Chile is the general paucity of carbonate minerals (for example, trona, calcite, and aragonite) which are widespread in other desert regions.Among the many morphological and s• tructural features that can be recognized in and near salars of northern Chile, the most unusual occur in hard rock-salt crusts, which in themselves are scarce in other arid regions. Included are features due to corrosion of rock-salt crusts by windblown water or free-flowing surface water, such as: ( 1) salt cusps and crenulate margins of salars, (2) salt channels, (3) salt pseudobarchans, and ( 4) salt tubes. Constructional features• in the salars include: ( 1) gypsum buttresses at borders of saline ponds, (2) salt veins, (3) salt stalactites, and (4) salt cones. In some salars, new fresh-water springs have formed steep-walled brine pools in thick rock-salt crusts. Prominent salt cascades and constructional salt terraces have been built up in one Andean valley by springs that are fed by brine from a nearby salar ( Salar de Pedernales).Sag basins and prominent scarps occur along faults that cut through the salt mass of Salar Grande.Of . the 67 closed basins in the An...

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Theory and general application of the Fraunhofer line-depth method for sensing luminescence • • • • • • The nat~~e of solar Fra~ofer lines General l'rinciple of the Fra~ofer.

Response of a Fraunhofer line discriminator (FLD) to varying distributions of granulated corncobs stained with varying concentrations of Rhodamine WT dye was tested on the ground and from an H-19 helicopter. By design the instrument detected fluorescence specifically at the sodium D^ Fraunhofer line (5890 angstroms). The granules are used as a vehicle for airborne emplacement of poison to control fire ants in the eastern and southeastern United States. The granules are dropped with considerable precision but some targets are inevitably missed. It was hoped that the FLD could aid in detecting missed target areas. Test results showed that the granules are detectable by FLD but that the concentration must be too great to be practical with the present FLD. Possible methods for enhancement of response may include: (1) increasing dye concentration; (2) incorporating with the poisoned gramiles a second material to carry the dye alone; (3) use of a more strongly fluorescent substance (at 5890 A); (4) modifying the time interval after dyeing, or modifying the inethod of dyeing;(5) modifying the FLD for greater efficiency, increased field of view (FOV), or larger optics; or (6) experimenting v/ith laser-stimulated fluorescence.

Although no fluorescence was detected, the mechanical operation of the FLD was satisfactory throughout 8 hours of flight, with attendant vibrations, and during a wide range of sun angles, from 13° to 33°. In accord with design, the FLD showed no appreciable response to moderate contrasts in reflectivity among targets, although there was a marked response to strong contrasts in reflectivity. Airborne tests also established that a minimum instrumental warm-up time of seven minutes is required, and that response is not entirely independent of either the vertical angle (tilt) or horizontal angle (flight direction) of the aircraft.