. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Kg' 720. the most part removed. The cur- rents of lava in Catalonia, like those of Auvergne, the Yivarais, Iceland, and all mountainous coun- tries, are of considerable depth in narrow defiles, hut spread out into comparatively thin sheets in places where the valleys widen. If a river has flowed on nearly level ground, as in the great plain near Olot, the water has only excavated a channel of slight depth; but where the declivity is great, the stream has cut a deep s
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. Kg' 720. the most part removed. The cur- rents of lava in Catalonia, like those of Auvergne, the Yivarais, Iceland, and all mountainous coun- tries, are of considerable depth in narrow defiles, hut spread out into comparatively thin sheets in places where the valleys widen. If a river has flowed on nearly level ground, as in the great plain near Olot, the water has only excavated a channel of slight depth; but where the declivity is great, the stream has cut a deep section, sometimes by penetrating directly through the central part of a lava-current, but. more frequently by passing between the lava and the secondary or tertiary rock which bounds the valley. Thus, in the accompanying section (fig. 721), at a. Conglomerate. 5. Thin seams of volcanic sand and scoriae. Fig. Section above the bridge of Cellent. d. Scoriae, vegetable soil, and alluvium. a. Scoriaceous lava. &. Schistose basalt. c. Columnar basalt. e. Nummulitic limestone. /. Micaceous gray sandstone. the bridge of Cellent, six miles east of Olot, we see the lava on one side of the small stream ; while the inclined stratified rocks consti- tute the channel and opposite bank. The upper part of the lava at that place, as is usual in the currents of Etna and Vesuvius, is scoria- ceous ; farther down it becomes less porous, and assumes a spheroidal structure; still lower it divides in horizontal plates, each about 2 inches in thickne-ss, and is more compact. Lastly, at the bottom is a mass of prismatic basalt about 5 feet thick. The vertical columns often rest immediately on the subjacent stratified rocks ; but there is sometimes an intervention of sand and scoriae such as cover the coun- try during volcanic eruptions, and which, unless protected, as here, by superincumbent lava, is washed away from the surface of the land. Sometimes the bed d contains a few pebbles and ang
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
