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The origin of such conglomerates is explained by observing the shingle beaches composed of trap- pebbles in modern volcanoes, as at the base of Etna. I shall now select examples of contemporaneous volcanic rocks of successive geological periods, to show that igneous causes have been in activity in all past ages of the world.
Islands of volcanic origin, not themselves volcanoes, but which have been probably recently upraised, are generally more or less completely surrounded by fringing reefs of coral, and have beaches of shining white coral sand. Their coasts present volcanic conglomerates, basalt, and in some places a foundation of stratified rocks, with patches of upraised coral.
I noticed also some contorted, highly inclined talcose schists, full of small quartz veins, generally running between the laminae of the schists. Probably the conglomerates had been produced by the wearing down of these schists. We passed through two Indian towns the first Yalaguina, the second Totagalpa.
The upper part of this great bed is slightly cellular; the lower part compact: the thickness varied a little in different parts. Manifestly a submarine lava; and is allied to bed 11. 16th and 17th. Dull purplish, calcareous, fine-grained, compact sandstones, which pass into coarse white conglomerates with numerous particles of quartz. 18th.
The older tertiary strata, forming the higher isolated hills and extensive tracts of country, vary, as I have said, extraordinarily in composition: within the distance of a few miles, I sometimes passed over crystalline limestone with agate, calcareous tuffs, and marly rocks, all passing into each other, red and pale mud with concretions of tosca-rock, quite like the Pampean formation, calcareous conglomerates and sandstones, bright red sandstones passing either into red conglomerate, or into white sandstone, hard siliceous sandstones, jaspery and chalcedonic rocks, and numerous other subordinate varieties.
Humboldt alludes to these formations repeatedly; it is true that he refers them to the ancient conglomerates of the Devonian age, but his description agrees so perfectly with what I have observed along the banks of the Amazons, that there can be no doubt he speaks of the same thing.
This alone convinced me beyond all dispute, that these books did not come from one and the same hand, but are conglomerates formed out of older materials, unartistically and mechanically joined. Indeed a fuller examination showed in Exodus and Numbers a twofold miracle of the quails, of which the latter is so told as to indicate entire unacquaintance with the former.
According to Professor Suess, the most ancient and purely marine of the Miocene strata in this basin consist of sands, conglomerates, limestones, and clays, and they are inclined inward, or from the borders of the trough towards the centre, their outcropping edges rising much higher than the newer beds, whether Miocene or Pliocene, which overlie them, and which occupy a smaller area at an inferior elevation above the sea.
To these conglomerates succeed argillaceous and calcareous marls and limestones, containing Lower Miocene shells and bones of mammalia, the higher beds of which sometimes alternate with volcanic tuff of contemporaneous origin.
The central plateau consists of ancient crystalline rocks with granites overlain by unfossiliferous sandstones and conglomerates considered to be of Palaeozoic age. The outcrops are largely hidden under laterite. The median zone is composed largely of crystalline rocks with granites and some Palaeozoic unfossiliferous rocks. The littoral zone contains the only fossiliferous strata.
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