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abundant accumulation acid alteration alumina augite basalt beds calcareous calcite calcium carbonate Carboniferous cavities character chemical clay cleavage colour compact composition cone consist constituents containing crater crystalline rocks crystals density deposits depth detritus diabase dolerite dykes earth eruption evidence exposed feet felsite felspar fissures foliated formations fragments geological geologists glass globe gneiss grains granite granular heat hornblende igneous iron lakes land lava layers lime limestone magnesia magnetite mass materials metamorphism mica microliths microscope miles mineral molten mountain observed occur ocean olivine organic original orthoclase oxide planet porphyritic portions present probably Q. J. Geol quartz quartz-porphyry regions rise river round sand sandstone schistose schists sediment sedimentary shale silica Silurian solid sometimes specific gravity steam stone strata stratified structure substance sulphur surface temperature terrestrial thick traced trachyte triclinic tuffs usually vapours varieties vast veins vents vitreous volcanic action volcanic rocks weathering
Popular passages
Page 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...
Page 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...
Page 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...
Page 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...
Page 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...
Page 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...
Page 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.
Page 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
Page 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...
Page 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...