Text-book of GeologyMacmillan and Company, 1893 - 1147 páginas |
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Términos y frases comunes
abundant accumulation acid alteration augite bands basalt beds Bull calcareous calcite Cambrian carbonate Carboniferous character chemical clay cone conglomerates containing crystalline rocks crystalline schists crystals denudation deposits depth Devonian diabase diorite dykes earth Edin eruptive rocks fauna feet felsite felspar fissures foliated formations fossils fragments genera Géol geological geologists glacier gneiss granite graptolites greywacke hornblende igneous Jahrb Journ lakes land lava layers lime limestone Lower mass materials metamorphism mica mica-schist microscope miles minerals mountain North observed occur ocean Old Red Sandstone olivine organic remains original Palæozoic porphyritic portions pre-Cambrian present probably Proc pyroxene Q. J. Geol quartz quartzite region rise river round sand schistose schists Scotland sediment sedimentary shales silicates Silurian Silurian rocks slates sometimes species stones strata stratified structure substance surface temperature terrestrial thick traced tuffs unconformably Upper Silurian vapours veins volcanic zone
Pasajes populares
Página 18 - ... axis of rotation, always very near one another, may have been in ancient times very far from their present geographical position, and may have gradually shifted through 10, 20, 30, 40, or more degrees without at any time any perceptible sudden disturbance of either land or water.
Página 22 - ... 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.
Página 3 - But it ought not to be allowed a firmer footing, nor on any account be suffered to blind us to the obvious truth 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.
Página 28 - N. lat., whereas the north-east trades seldom blow south of the equator. The effect of the northern trades blowing across the equator to a great distance will be to impel the warm water of the tropics over into the Southern Ocean. But this is not all ; not only would the median line of the trades be shifted southwards, but the great equatorial currents of the globe would also be shifted southwards.
Página 467 - before the sea, advancing at a rate of ten feet a century, could pare off more than a mere marginal strip of land between 70 and 80 miles in breadth, the whole land might be washed into the ocean by atmospheric denudation...
Página 16 - If, therefore, the axis of the earth were perpendicular to the plane of its orbit, the days and nights would...
Página 27 - Snow and] ice lower the temperature by chilling the air and condensing the vapour into thick fogs. The great strength of the sun's rays during summer, due to his nearness at that season, would, in the first place, tend to produce an increased amount of evaporation. But the presence of snowclad mountains and an icy sea would chill the atmosphere and condense the vapour into thick fogs. The thick fogs and cloudy sky would effectually prevent the sun's rays from reaching the earth, and the snow, in...
Página 446 - Eddystone lighthouse, during a storm in 1840, a door which had been securely fastened against the force of the surf from without, was actually driven outward by a pressure acting from within the tower, in spite of the strong bolts and hinges, which were broken. We may infer that, by the sudden sinking of a mass of water hurled against the building, a partial vacuum...
Página 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...
Página 17 - Now mark the answer to this question, for it contains the whole doctrine of the seasons THE AXIS OF THE EARTH IS INCLINED TO THE PLANE OF ITS ORBIT ; AND IT MOVES ROUND THE SUN IN A DIRECTION PARALLEL TO ITSELF.