OCR Text |
Show 188 OLDEl~ PLIOCENE l'.ElUOD. (Ch.XlV paratively 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 rock which bounds the valley. 'Thus, in the accompanying section, at the bridge of Cellent, six miles east of Olot, we sec the lava on one side of a, Scoriaceous lava. b, Schistose basalt. c, Columnar basalt. No.45. Section above t!le bridge of Cellent. rl, Scoriro, vegetable soil, and alluvium, e, Nummulitic limestone. f, Micaceous grey sandstone. the small stream, while the inclined stratified rocks constitute the channel and opposite bank. The upper part of the Java at that place is scoriaceous; farther down it becomes less porous, and assumes a spheroidal structure ; still lower it divides in horizontal plates, each about two inches in thickness, and is more compact. Lastly, at the bottom is a mass of prismatic basalt about five feet thick. The vertical columns often rest immediately on the subjacent secondary rocks; but there is sometimes an intervention of such sand and scorire as cover a country during volcanic eruptions, and which when unprotected, as here, by superincumbent lava, is washed away from the surface of the land. Sometimes the bed d contains a few Ch. XlV.) RAVI~ES T~ LAVA l::XCAYATED BY RI\"ERS. 1S9 pebbles and angular fragments of rock ; in other places une earth, which may have constituted an ancient vegetable soil. In several localities, beds of sand and ashes are interposed between the lava and subjacent stratified rock, as may be seen if we follow the course of the Java-current which descends from Las Planas towards Amer, and stops two miles short of that town. The river there has often cut through the lava, and through. eighteen feet of underlying limestone. Occasionally an alluviUm, several feet thick, is interposed between the igneous and marine formation ; and it is interesting to remark, that in this, as in other beds of pebbles occupying a similar position, there are no rounded fragments of lava, whereas, in the modern gravel beds of rivers in this country, volcanic pebbles are abundant. 'l'he deepest excavation made by a river through lava, which I observed in this part of Spain, is that seen in the bottom of a valley near San Feliu de Pallerols, opposite the Castell de Stolles. The lava there has filled up the bottom of a valley, and a narrow ravine has been cut through it to the depth of 100 feet. In the lower part the lava has a columnar structure. A great number of ages were probably requit·ed for the erosion of so deep a ravine ; but we have no reason to infer that this current is of higher antiquity than those of the plain near Olot. The fall of the ground, and consequent velocity of the stream being in thi~ case greater, a more considerable volume of rocl~ may have been removed in an equal quantity of time. We shall ~esc~·ib: one more section to elucidate the ph e. nomena of this d1str1ct. A lava-stream, flowing from a ridcre of .hi~ls to the east of Olot, descends a considerable slo~e untl!It reaches the valley of the river Fluvia. Here, for the first time, it comes in contact with running water, which has removed a portion, and laid open its internal structure in a precipice about 130 feet in height, at the edge of which stand: the town of Castell Follit. By the junction of the rivers Flu via and Teronel the mass of lava has been cut away on two sides; and the insular mass |