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Show 158 OLDER PLIOCENE PERIOD· [Ch. XII. larly stratified. He seems to have been _misled by _Brocchi's description, who contrasts the more crystallme and sohd texture of the older secondary rocks of the Apennines w~th the loose and incoherent nature of the Subapennine beds, wluch resemble, he says, the mud and sand now deposited by the sea.. . . We have endeavoured, in the last chapter, to restrict wlthm definite limits the meaning of the term alluvi'um ; but if the Subapennine beds are to be design.ated 'mari~e alluvia,' the same name might, with equal propnety, be apphed not only to the ar()'illaceous and sandy groups of the London and Hampshire basins, but to a very great portion of our secondary series where the marls, clays, and sands are as imperfectly consolidated as the tertiary strata of Italy in general. They who have been inclined to associate the idea of the more stony texture of stratified deposits with a comparatively higher antiquity, should consider how dissimilar, in this respect, are the tertiary groups of London and Paris, although admitted to be of contemporaneous date, or they should visit Sicily and behold a soft brown marl, identical in mineral character with that of the Subapennine beds, underlying a mass of solid and regularly-stratified limestone, rivalling the chalk of En()'land in thickness. This Sicilian marl is older than the b superincumbent limestone, but newer than the:Subapennine marl of the north of Italy ; for in the latter the extinct shells rather predominate over the recent, in the former the recent predominate almost to the exclusion of the extinct. We shall now consider more particularly the characters of those Subapennine beds which we refer to the older Pliocene period. Subapennine marls.-The most important member of the Subapennine formation is a marl which varies in colour from greyish brown to blue. It is very aluminous, and usually contains much calcareous matter and scales of mica. It often exhibits no lines of division throughout a considerable thickness, but in other places it is thinly laminated. Near Parma, for example, I have counted thirty distinct laminre in Ch. XII.] SUBAP.ENNINE MARLS, 159 the thickness. of an inch: In some of the hills near that city the marl attams, accordmg to Signor Guidotti, a thickness of near]~ 2000 feet, and. is ch~rged throughout with shells, many of whiCh are such as mhabit a deep sea. They often occur in layers in such a manner as to indicate their slow and gradual accumulation. They are not flattened but are filled with marl. Beds of lignite are sometimes interstratified, as at Medesano, four leagues from Parma; subordinate beds of gypsum also occur in many places, as at Vigolano and Bargone, in the territory of Parma, where they are interstratified with shelly marl an~ sa~d. At ~ezig~ano, in the Monte Cerio, the sulphate of hme Is found m lenticular crystals, in which unaltered shells are sometimes included. Signor Guidotti, who showed me specimens of this gypsum, remarked, that the sulphuric acid must have been fully saturated with lime when the shells were enveloped, so that it could not act upon the shell. Accordin(JI to Brocchi, the. marl sometimes passes from a soft and pulveru~ ~cnt s_ubst~nce mto a compact limestone*, but it is rarely found m thts solid form. It is also occasiona1ly interstratified with sandstone. 'l'he marl constitutes very frequently the surface of tl h · 1e coun~ry, . avmg. no covering of sand. It is sometimes seen reposm~ tmmedtately ~n the A ~ennine limestone; more rarely g~avel Intervenes, as m the htlls of San Quirico t. Voica ·m c Tro cks are here and there superimposed ' as at R d' £ · a tco am, In u.scany, where a hill composed of marl, with some few she~ls mter~persed, is capped by basalt. Several of the volcamc tuffs m the same place are so interstratified with the marls as to show that the eruptions took place in the sea durin()' the older Pl~ocene period. At Acquapendente, Viterbo, and ~ther places, htlls of the same formation are capped with trachytic lav d · h lr!. • a, an Wit tuus whtch appear evidently to have been sub-aqueous. Yellow Sand.-The other member of the S b . gro tl 11 u apennmc up, le ye ow sand and conglomerate, constitutes, in most • Conch, Foss. Subap., tom. i. p. 82. t Ibid., p. 78. |