Experiments in connection with Daly's hypothesis on the formation of submarine canyons

Shepard has made us familiar with the shape and various properties of submarine gorges and in a number of publications he has shown the great importance to earth-science of these wonderful features of the sea floor. Various hypotheses have been brought forward to explain the formation of submarine canyons. The only hypothesis to which there are no grave objections, is that suggested by Daly. During the low sea levels of the ice age mud was stirred up on the shallowing shelves and the dense water ensuing, flowed down the continental slopes, gradually eroding the gorges. This theory can be substantiated by experiments. These proved that a suspension will flow down a slope without being much diluted by mixing, that such a flow will concentrate in slight depressions in the flat and follow a rut down the slope, that the current will begin to form ripple marks in sand at a velocity of 10—20 cm/sec and that the speed varies with the root of the density and probably also roughly with the root of the depth. Rough estimations, based on the data of one of the Georges Bank canyons, show that the velocity in nature should be somewhat less than that of larger rivers, but encreasing as it proceeds and catches up more sediment. At the lower end the suspension has gained 6 times the (effective) density and two to three times the velocity it started with. At this stage the amount of sediment has caught up the average of most larger rivers and reached that of the most muddy major rivers. Some 10.000 flows must have occurred, or one every 5 years, each eroding as much sediment as the Mississippi carries to sea in two or three weeks. The current lasted many hours and its discharge was several times the average of the Mississippi. The relatively large percentage of materials carried by the currents is a consequence of its having slumped down the sides of the gorge before the current picked it up. However rough these figures may he, they show that no preposterous assumptions have to be made to arrive at reasonable relations between velocities, amounts of water and sediment and size of the gorges for Georges Bank. Where the shelf is narrow, as at some of the Californian canyons, there appears to have been hardly enough sediment to set up the necessary currents. The chief remaining uncertainty lies in the necessity of assuming small strength of the rocks forming the continental slopes. But all evidence available except some of Shepard's dredging results, points to the same conclusion. Older, smothered gorges may have been cleaned out by this mechanism. In any case, as Stetson points out, the mechanism must have played an important part in the sedimentation on and beyond the continental slope. Groningen, March 1937. An 8-mm film illustrating the experiments, of about 20 meters length, can be obtained at the price of 8 guilders from the Rijksmuseum van Geologie at Leyden

The negative isostatic anomalies in the East Indies (with Experiments)

In this paper a series of experiments is described relating to the deformation of the earth's crust by horizontal compressive stress. A floating model crust is compressed in most cases, after a slight dent has been run across it to represent a geosyncline. When materials of the correct order of strength are used, the crust buckles down below the model geosyncline forming a root at the lower surface of the crust. At no time in this process is a topographic depression formed at the surface that exceeds the depth of deep-sea troughs as compared to the thickness of the earth's crust. Other possibilities suggested by the experiments are that the crust may break through and overthrust and that a broad geosyncline will surmount a more complicated form of root. It was also found that the direction of compressive stress need not be at right angles to the course of the geosyncline to produce a root and that an orogenetic belt need not be more plastic than the remainder of the crust, but that the contents of the geosyncline must have less aggregate strength than a layer of the crust itself of the same thickness, as they would otherwise disappear down into the root. These experiments illustrate and clarify the theories of crustal buckling evolved by Vening Meinesz to account for the anomalies of gravity in the East Indies and by Bucher from geological data. In the second part of this paper an attempt is made to explain the recent isostatic history of the East Indies on the basis of the buckling hypothesis. The chief aim is to show that the anomalies may date back to the miocene orogenetic phase, and that the recent vertical movements can be looked upon as a slow adjustment to regain isostatic equilibrium. Geological Laboratory, Groningen

Petrografic description of some igneous rocks from the Cordillera of South Mendoza, in the Argentine

Comparatively little is yet known of the intricate igneous history of the Cordillera of South Mendoza. Almost all the knowledge we have is due to the field- and stratigraphical work of Dr. H. Gerth of Leiden (bibl. 1 and 2), and the petrographic studies of Dr. H. G. Backlund (bibl. 3 and 4) on the material collected by the former investigator. A part of this material, however, had remained unexamined at Leiden, and at the request of Dr. Gerth Dr. L. U. de Sitter began an examination of the 25 slides prepared. After a provisional examination and the determination with the universal stage 1) of some plagioclases in most of the slides that were not too much decomposed Dr. de Sitter left Leiden for Dutch East India. Dr. Gerth was so kind as to allow me the further examination, and I have great pleasure in expressing my sincere thanks to him for entrusting me with the valuable material. All data concerning the geological occurrences and field-evidence were supplied by Dr. Gerth. I myself, not having the use of a theodolite microscope for the time being, Mr. Ch. Harloff was so good as to make some further additions to Dr. de Sitter's determinations (slides n°. 970, 9, and 28, with the accompanying drawings). The petrographic descriptions are on my own responsability. In a few cases I thought it better to give other names to rocks, that were identical with, or very closely related to specimens already described by Backlund, taking Holmes’ “Nomenclature of petrology” as a basis. For a pre-tertiary andesite the somewhat antiquated term of “porphyrite” has not been employed. For the micrographs I used rough tracings made with an optical bench for micro-projection. The general shape and relative sizes of the larger items are therefore accurate

Experiments on the formation of windworn Pebbles

The influence of the wind laden with sand in modelling pebbles is believed by some authors to be only that of polishing the surface, by others of rounding off bits of stone that already possessed edges and corners, or again by others of wearing any fragment either rounded or angular into definite forms with ridges and facets, dependent on the shape of the basis (Alb. Heim). Experiments, fully confirming the last opinion, are described in this paper: no rounding off took place, while the models were slowly revolved in the sandblast, and vertical planes took on a backward slanting position, cutting eachother along sharp edges. Where sand corrosion is great, as in the desert, the windworn pebbles owe their shape to the laws formulated by Heim; many of the fossil windworn pebbles of Northern Europe have undergone but slight alteration from their original shape and size by the natural sandblast, others seem to have been entirely remodelled by the wind along the lines indicated above

Experiments in connection with Salt Domes

The different theories concerning the origin of Salt Domes in Roumania, Germany, Texas, Louisiana, Colorado and Utah are discussed. In Roumania the salt occurs in cores of “Diapir” anticlines. The existance of hills of salt indicates, that the salt is still pushing upwards. In Germany the salt district shows slight folding but the salt itself is intensively folded. The theory of Lachmann-Arrhenius-Harbort explains the salt domes by isostasy combined with a lower specific gravity and greater plasticity of the salt than of the covering layers. This theory is opposed by Stille, who accounts for the salt domes by mesozoic folding. The latter theory has apparently gained preference in America for the explanation of the Salt Domes in Texas and Louisiana, although no indications of folding are met with there. Two series of experiments were carried out. Those of the first series were made to determine the form in three dimensions of the intricate folding, observed in the German salt mines and of which the folds round vertical axes in particular are very remarkable. Further it was the aim to find out if, by making use of forces, which may be compared to isostasy, similar folds could be artificially produced. When using layers of identical plasticity, regular congruous folds occurred (exp. I, 1). When layers of different plasticity alternated with eachother, smaller complicated, dis-harmonious folds arose, superimposed upon larger ones, corresponding to those of the preceeding experiment (exp. I, 2—7). It is important to note that in the field of vertical pressure, by difference in the viscosity between plastic and less plastic material, fissures were torn in the less plastic material, at right angles to the direction of movement, which were filled up by the plastic material. Similar rents may be expected in the salt fields. The experiments of the 2d series were made with a counter pressure equal to half the initial pressure per surface unit. The reason for making these experiments was that in the first series air-spaces occured. In nature, also, a considerable counter-pressure exists, during the rising of the salt in consequence of the weight of the covering layers. Remarkable folds were formed, which, in material of identical plasticity, showed an M-form in vertical section (exp. 1, series II). Exp. 6, series II showed that with a thick series of layers the top layers may begin to move before the lower ones. In this way two M-shapes originated one above the other. Exp. 3 and 4 were made with white paraffin of uniform melting point in which were placed a horizontal row of vertical pillars divided into layers, so as to be able to reconstruct the stream lines of the paraffin. Here the friction between the paraffin and the iron walls of the compression apparatus were seen to exercise an important influence upon the movement of the paraffin. The principle result of the experiments is that all shapes of folds, observed in the German salt domes, can be completely explained by Lachmann’s theory, that is by the isostatic pressing up of the specifically lighter salt in pillar-like masses. This alone, however, does not exclude the possibility that tangential pressure may be partially or entirely responsible for the known phenomena. The senior author gave a lecture on the first series of experiments at Bâle on September 3, 1927, at a meeting of the Mineralogical section of the Schweiz. Naturforschende Gesellschaft (bibl. 19) and at Delft in the annual meeting of the geological section of the Geol. Mijnbouwkundig Genootschap voor Nederland en Koloniën in March 1928