Textbook of GeologyMacmillan, 1882 - 971 стор. |
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Загальні терміни та фрази
abundant accumulated acid altered Ammonites appear augite bands basalt basin beds brachiopods calcareous calcite carbonate Carboniferous chalk character chemical chiefly clay coal cone conglomerates consist containing corals Cretaceous crust crystalline rocks crystals denudation deposits depth detritus Devonian dolomite earth Eocene eruptive Europe evidence fauna feet thick felspar fissures foliated formations forms fossils fragments genera geological geologists glaciers gneiss granite graptolites grey heat hornblende igneous iron lakes land lava layers lime limestone Lower Silurian marine marls mass metamorphism microliths microscope miles mineral Miocene mountain North numerous observed occur ocean Old Red Sandstone organic remains original orthoclase oxide period Permian plants porphyritic portions present Q. J. Geol quartz region river round sand schists Scotland sediment sedimentary shales shells siliceous Silurian sometimes species stone strata stratified structure substance surface temperature terrestrial Tertiary traced tuffs unconformably Upper Silurian valley vapours veins zone
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Сторінка 21 - ... of the earth must have been continually diminishing, and thus the Polar regions must have been ever rising and the equatorial ones falling ; but, as the ocean always followed these changes, they might quite well have left no geological traces. "The tides must have been very much more frequent and larger, and accordingly the rate of oceanic denudation much accelerated. " The more rapid alternation of day and night...
Сторінка 502 - An important feature in the joints of stratified rocks is the direction in which they intersect each other. In general they have two dominant trends, one coincident, on the whole, with the direction in which the strata are inclined from the horizon, and the other running transversely at a right angle or nearly so. The former set is known as dip-joints...
Сторінка 442 - the mere consideration of the number of cubic feet of detritus annually removed from any tract of land by its rivers does not produce so striking an impression upon the mind as the statement of how much the mean surface level of the district in question would...
Сторінка 451 - The sea has really had less to do than the meteoric agents. A ' plain of marine denudation' is that sea-level to which a mass of land had been reduced mainly by the subaerial forces, the line below which further degradation became impossible, because the land was thereafter protected by being covered by the sea. Undoubtedly the last touches in the long process of sculpturing...
Сторінка 3 - ... him enormous labour. It is also notable for the qualification which he attached to Huttonian teaching, as he thought uniformitarianism had been pushed too far. " It has often been insisted upon that the Present is the key to the Past ; and in a wide sense this assertion is eminently true. . . . While, however, the present condition of things is thus employed, we must obviously be on our guard against the danger of unconsciously assuming that the phase of nature's operations which we now witness...
Сторінка 501 - ... intersection. As a rule, they are most sharply defined in proportion to the fineness of grain of the rock. In limestones and close-grained shales, for example, they often occur so clean-cut as to be invisible until revealed by fracture or by the slow disintegrating effects of the weather. The rock splits up along these concealed lines of division whether the agent of demolition be the hammer or frost. In coarse-textured rocks, on the other hand, joints are apt to show themselves as irregular...
Сторінка 3 - ... that the few centuries wherein man has been observing nature form much too brief an interval, by which to measure the intensity of geological action in all past time. For aught we can tell the present is an era of quietude and slow change, compared with some of the eras that have preceded it.
Сторінка 502 - ... rock between them, when seen from a distance, appear like so many wall-like masses. An important feature of these joints, as mentioned by this authority, is the direction in which they intersect each other. In general they have two dominant trends, one coincident on the whole with the direction in which the strata are inclined from the horizon, and the other running transversely at a right angle, or nearly so. The first are called "dip joints
Сторінка 25 - This is not all ; the winters would then not only be colder than now, but they would also be much longer. At present the winters are nearly eight days shorter than the summers ; but with the • eccentricity at its superior limit and the winter solstice in aphelion, the length of the winters would exceed that of the summers by no fewer than thirty-six days. The lowering of the temperature and the lengthening of the winter would both tend to the same effect, viz., to increase the amount of snow accumulated...
Сторінка 234 - ... mud, which, with various gases, is continuously or intermittently given out from the orifice or crater in the centre. They occur in groups, each hillock being sometimes less than a yard in height, but ranging up to elevations of 100 feet or more. Like true volcanoes, they have their periods of repose, when either no discharge takes place at all, or mud oozes out tranquilly from the crater, and their epochs of activity, when large volumes of gas, and sometimes columns of flame, rush out with considerable...