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Show CHAPTER XXIII. Secondary formations-Brief enumeration of the principal groups-No species common to the secondary and tertiary rocks-Chasm between the Eocene and Maestricht beds-Duration of seconuary 1>erious-Former continents !>laced where it is now sea-Secomlary fresh-water deposits why rare-Persistency of mineral composition why apparently greatest in older rocks-Supposed univer. sality of red marl formations-Secondary rocks why more consolidated-Why more fractured and disturbed-Secondary volcanic rocks of many different ages. SECONDARY FORMATIONS. As we have already exceeded the limits originally assigned to this worl{, it is not our intention to enter, at present, upon a detailed description of the formations usually called ' Secondary,' the elucidation of which might well occupy another volume. By 'secondary,' we mean those stratified rocks older than the tertiary, which contain distinct organic remains, and which sometimes pass into the strata called 'Primary,' to be described in our concluding chapters. The observations which we are about to offer have chiefly for their object to show that the rules of interpretation adopted by us for the tertiary formations, are equally applicable to the phenomena of the secondary series. ~his last has been divided into several groups, and we shall briefly enumerate some of the principal of these for the convenience of 1:eferen~e, w~thout pretending to offer to the student ~ syste~at1c class1ficat10n, founded on a full comparison of fossil remams. PRINCIPAL SECONDARY GROUPS. (Deacending Series.) 1. Strata from the chalk of Maestricht to the lower green-sand inclusive. The number of species of testacea already procured from the different members of this division amount to about 1000. The principal subdivisions are the Maestricht beds, the chalk with and without flints, the upper green-sand, the gault, and Ch. XXIII.] MAESTRlCHT AND WEALDEN BEDS. 325 the lower green-sand. The nrst of these groups is seen at St. Peter's Mount, Maestricht, reposing upon the upper flinty chalk of England and France. It is characterized by a peculiar assemblage of organic remains, perfectly distinct from those of the tertiary period. M. Deshayes, after a careful comparison, and after making drawings of more than 200 species of the Maestricht shells, has been unable to identify any one of them with the numerous tertiary fossils in his co1lection. On the other hand, there are several shells which are decidedly common to the calcareous beds of Maestricht and the white chalk. The names of twelve of these, communicated by M. Deshayes, will be found in Appendix II., p. 60. But the fossils of the Maestricht beds extend not merely into the white chalk of the French geologists, but into their 'green-sand,' which appears to correspond Yery nearly with the upper green-sand of the English geologists. A list of £ve species of shells, common to the Maestricht beds and the upper green-sand of France, will be found in Appendix II., p. 60. It will be seen by the above lists, that the belemnite, one of the cephalopodes not found in any tertiary formation, occurs in the Maestricht beds; an ammonite has also been discovered in this group by Dr. Fitton, and is now in the collection of the Geological Society of London. That gigantic species of reptile, the Mososaurus of Maestricht, has also been found by Mr. Mantell in the English chalk. 2. The Wealden, or the strata from the Weald clay to the Purbeck limestone inclusive. The numerous fossil-shells of this group are referrible to freshwater genera, which are associated with many remains of fluviatile and terrestrial reptiles and land-plants. We believe that no species, whether animal or vegetable, in this group, has been distinctly identified with any found either in the superincumbent marine beds of the nrst division, or in the subjacent rocks of the group No. 3, which are also of marine origin. |