. Elements of geology : a text-book for colleges and for the general reader. Geology. 602 CEXOZOIC ERA—AGE OF MAMMALS. ent river-beds, R'. In this locality G represents the deep placers, and G' the superficial placers. The history of changes shown in these sections is sufficiently obvi- ous. In the time of the old river-system, R was a river-bed, doubtless with a ridge on either side represented by the dotted lines. In this bed accumulated gravel, containing gold. Then came the lava-flow, which of course ran down the valley, displacing the river and covering up the gravels. The displaced river
. Elements of geology : a text-book for colleges and for the general reader. Geology. 602 CEXOZOIC ERA—AGE OF MAMMALS. ent river-beds, R'. In this locality G represents the deep placers, and G' the superficial placers. The history of changes shown in these sections is sufficiently obvi- ous. In the time of the old river-system, R was a river-bed, doubtless with a ridge on either side represented by the dotted lines. In this bed accumulated gravel, containing gold. Then came the lava-flow, which of course ran down the valley, displacing the river and covering up the gravels. The displaced rivers now ran on either side of the resistant lava, and cut out new valleys, 2,000 feet deep, in the solid slate, leaving the old lava-covered river-beds and their auriferous gravels high up on a ridge. The deeper cutting was the result of the higher slope. In other cases the convulsion which ejected the lava also changed greatly the direction of the slope of the country, and there- fore the direction of the streams. In such cases of course the present river-system cuts across the old river-beds and gravels, and their cover- ing lavas, as shown in Fig. Fig. 966.—Lava-Stream cut through by Rivers: a, a, basalt; b, b, volcanic ashes; c, c. Tertiary; d, d. Cretaceous rocks: i?, i?,"direction of the old river-bed; R', R', sections of the present river-beds (.from Whitney). Age of the River-Gravels.—The age of the old river-gravels is still doubtful; that of the newer river-gravels is undoubtedly Champlain or early Terrace. Below we give a list, taken from Whitney, of the re- mains found in these gravels : f Great mastodon. | Mammoth. x- , Bison. Is ewer placers. Horse, modern. [_ Man's works. f Great mastodon.* Mammoth. Deep placers. Mylodon. Tapir, modern. Rhinoceros (ally). Hippopotamus (ally). Camel (ally). Horse, extinct species. It will be seen that the fauna of the deep placers unite Pliocene and Quaternary characters. The great mastodon, the mammoth, the * Whitn
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
