OCR Text |
Show CHAPTER XVII. Imbeuuing oi aquatic species in subaqueous strata-Inhumation of iresh-wafef plants and animals-Shell marl-Fossilized seed-vessels and stems of Chant -Recent deposits in the American lakes-Fresh-water species dl"ifted into seas and estuaries-Lewes levels-Alternations of marine and fresh-water shata, how caused-Imbedding of marine plants and animals-Cetacea stran<le<l on our shores-Their remains should be more conspicuous in marine alluvium than the bones of land quadrupeds-Liability of littoral and estuary testacea to be swe1)t into the deep sea-Effects of a storm in the Frith of Forth-Burrowing shells secured from the ordinary action of waves and currents-Living testacea found at .considerable depths. WE have hitherto treated of the imbedding of terrestrial plants and animals, and of human remains in the deposits that are now forming beneath the waters, and we come next to consider in what manner aquatic species may be entombed in strata, formed in their own element. Imbedding of Fresh-wate·r Plants and Animals. The remains of species belonging to those genera of the animal and vegetable kingdoms, which are more or less exclu· sively confined to fresh-water, are for the most part preserved in the beds of lakes or estuaries, but they are oftentimes swept down by rivers into the sea, and there intermingled with the ex~vire of marine races. The phenomena attending their in· humation in lacustrine deposits, may be sometimes revealed to our observation by the drainage of small lakes, such as are those in Scotland which have been laid dry for the sake of obtaining shell marl for agricultural uses. In these recent formations, as seen in Forfarshire, two or three beds of calcareous marl are sometimes observed separated from each other by layers of drift peat, sand, or fissile clay. The marl often consists almost entirely of an aggregate of shells of the genera limnea, planorbis, valvata, and cyclas, with some few others, species of all which now exist in Scotland. A considerable proportion of the testacea appear to have died Ch. XVII.] ~78 very youn()', and few of tl 1 ll th . h ~ le s 1e s m·e of a size which indicates e1r avmO' attai d • o . ne a state of maturity. The shells are sometimes entirely dec d ./! • · ompose 'wrmmg a pulverulent marl· sometimes t.h ey are in a state of goo d preservati.o n. They are' frequently mte. rmixed with stem s of c1 la rre and other aquatic vege. tables, which a. re matted toOo 'ethe ran d compressed, form.m g lammre often as tlun as paper. A. s •t he chara. is an aquatic plant' w hI' c h occurs frequently fossilm formatiOns of different eras 'an d 1.: :~> of ten of much . 11114 portance to the geoloo·ist in charact · . . o enzmg entire groups of strata, we shall de• scribe th. e manner in w hI' c h t h e recent speci.e s have been found m. a p. etrified state · Tl1 ey occur m· one of the lakes. of Forfarsh1re, mclosed in nodul es, an d someti.m es m. a contmuous stratum of a kind of travertin. The seed-ve.s sel of these plants is remarl(abl y t oug11 an d hard, and con.s ists of a membranous nut covere-d by an m. tegu4 men.t (fig. d dlagt:am No.~,) both of which are spirally striated or ribbed. The mte.~..... ument is· composecl of fi1 ve sp1. ra l va1 v es, (No.2.) () 0 !J :f S01d-ue•ul of Ohara l•upitla, (a) Part of the stem with the seed-vessel attached Magnified. (b) Natural size of the seed-vessel. ' (c) Inte~umen~ of the Gyrogonite, or petrified seed-vessel of Charn . ound Ill the Scotch marl-lakes. Magnified. (tl) ~echon showing the nut within the integument. (Ve) ower end of the integument to which the stem was attached. OJ., II. T hispids, |