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Show 334 SECONDARY FORMATIONS [Ch. XXIII, Secondary rocks why more consolidated.-One o: the points where the analogy between the secondary and tertlat·y formations has been supposed to fail is the greater degree of solidity observable in the former. Undoubtedly the older rocks, in general, are more stony than tl:e newer ; ~nd m.ost of the tertiary strata are more loose and mcoherent m their texture than the secondary. Many exceptions, however, may be pointed out, especially in those calcareous and siliceous deposits ~vhich have been precipitated in great part from the wat~rs of m~ne.ral springs, and have been originally ~~mpact. Of th1s desc~Ipbon are a large proportion of the Par1s1an Eocene rocks, wlnch are more stony than most of the English secondary groups. But a great number of strata have evidently been consolidated subsequently to their deposition by a slow lapidifying rocess. 'rhus loose sand and gravel at·e bound together by ~aters holding carbonate and oxide of iron, carbonate of lime, silica, and other ingredients, in solution. These waters percolate slowly the earth's crust in different regions, and often remove gradually the component elements of fossil organic bodies, substituting other substances in their place. It seems, moreover, that the draining off of the waters during the elevation of land may often cause the setting of particular mixtures, in the same manner as mortar hardens when desiccated, or as the recent soft marl of Lake Superior becomes highly indurated when exposed to t~e air*· The conve.rsion of clay into shale, and of sand mto sandstone, may, m many cases, be attributed to simple pressure, produced by ~he weight of superincumbent strata, or by the upwar.d heavmg of subjacent masses during earthquakes. Heat IS .anoth~r c ause of a more compact and crystalline texture, wluc. h wrl,l be considered when we speak of the strata termed ' pt:Ima~y. All the changes produced by these various m:ans req~nre hme for their completion ; and this may explain, It~ a satisf~ctory manner, w h y the Older rocks are most consolidated, without * Vol. i. p. 226, and Second Edition, P· 259. Ch. XXIII.] WilY MORE DISTURBED. 335 .entitling us ~o re 1 sort to any hypothesis respecting an original distinctness In t 1e degree of 1apidi6cation of the secondary strata. Secondary rocks why more disturbed.-As the older formations are generally more stony, so also they are more fractured, curved, elevated, and displaced, than the newer. Are we, then to infer, with some geologists, that the disturbing forces wer~ more energetic in remoter ages? No conclusion can be more unsound; for as the moving power acts from below, the newer strata cannot be deranged without the subjacent rocks ar- .. . . I p tic1patmg m t 1e movement; while we l1ave evidence that the older have been frequently shattered, raised, and depressed, again and again, before the newer rocks were formed. It is evident that if the disturbing power of the subterranean causes be exerte~ with unifor~ intensity in each succeeding period, the qua~t1ty of convulsion u~dergone by different groups of strata Will ~enera~Jy be great .m proportion to their antiquity. But exceptiOns Will occur, owmg to the partial operation of the volcanic forces at particular periods, so that we sometimes find tertiary strata more elevated and disturbed, in particular countries, than are the secondary rocks in others. Some of the enormous faults and complicated dislocations of the ancient strata may probably have arisen from the continued repetition of earthquakes in the same place, and sometimes from two dis~inc~ series of convulsions, which have forced the same mas~es m different, and even opposite directions, sometimes by vertical, at others by horizontal movements. Secondary volcanic rocks oF dill'erent ages The a · t' '.1 ':U' ·- SSOCia Ion of volcanic rocks with different secondary strata is such as to pro~e, that there were igneous eruptions at many distinct perwds, as also that they were confined during each epo h r . d c J as now, to Imite areas. Thus, for example, igneous rocks con-tempo~ aneous with the carboniferous strata abound in some countnes, but are wanting in others. So it is evident that the botto~ of the sea, on which the oolite and its contemporary deposits were thrown down, was, for the most part, free from |