534 



PLIOCENE VOLCANOS. 



[Ch. XXXI. 



Fig. 



a. Secondary conglomerate. 



6. Thin seams of volcanic sand and scoriae. 



of the ejections composing the cones in these two regions. In the Eifel, 

 the quantity of pieces of sandstone and shale thrown out from the vents 

 is often so immense as far to exceed in volume the scorias, pumice, and 

 lava ; but I sought in vain in the cones near Olot for a single fragment 

 of any extraneous rock; and Don Francisco Bolos, an eminet botanist 

 of Olot, informed me that he had never been able to detect any. Vol- 

 canic sand and ashes are not confined 

 to the cones, but have been some- 

 times scattered by the wind over the 

 country, and drifted into narrow val- 

 leys, as is seen between Olot and 

 Cellent, where the annexed section 

 (fig. 668) is exposed. The light 

 cindery volcanic matter rests in thin 

 regular layers, just as it alighted on 

 the slope formed by the solid conglomerate. No flood could have passed 

 through the valley since the scoriae fell, or these would have been for 

 the most part removed. 



The currents of lava in Catalonia, like those of Auvergne, the Viva- 

 rais, Iceland, and all mountainous countries, are of considerable depth in 

 narrow denies, but 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 tbe 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. 669, at 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 constitute 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 scoriaceous ; farther down it becomes less porous, and assumes a sphe- 

 roidal structure ; still lower it divides in horizontal plates, each about 2 

 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 stratified rocks ; but there is sometimes 

 an intervention of sand and scoria? such as cover the countiy during vol- 

 canic 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 angular fragments of rock ; in other places 

 fine 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 lava-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 18 feet of underlying limestone. 

 Occasionally an alluvium, several feet thick, is interposed between the 



