| OCR Text |
Show ERA OF THE and Russia. The same system exists in No!· wh America, and p~·obably in other parts of t~e earth n.ot .yet geo~ogically Investigated Being a manne deposit, It establishes that seas exi~ted at the time of its formation on the tracts occupied uy it, while some of its organic remains prove that, in the neighborhood of those seas, there were tra.cts of dry land. The cretaceous fm·mation in England presents beds chiefly sandy in the lowest part, chiefly in the clayey in the middle, and chieflv of chalk in the upper part, the chalk beds being never absent, which some of the lower are in several places in the vale of the Mississippi ; again the true chalk is wholly, or all but wholly, absent. In the south of England, the lower beds are, (reckoning from the lowest upwards,) 1. Shankland or greensand, H a triple alternation of sands and sandstones with clay;'' 2. Galt, " a stiff blue or bl1c1ck clay, abounding in shells, which frequently possess a pearly lustre;" 3. Hard, chalk; 4 Chalk with flints; these two last being generally white, but in some districts red, and in others yellow. The v1hole arc, in En~land, about 1200 feet thick, showing the considerable depths of the ocean in which the deposits were made. Chalk is a carbonate of litne, ancl the manner of its production in such vast quantities \\-·as long a subject of speculation among geologists. Son1e light seemed to be thrown upon the subject a few years ago, when it was observed, that the detritus of coral reefs in the present tropical seas gave a powder, undistinguishabte, when dried, from ordinary chalk. It then appeared likely that the chalk beds were the detritus of the corals which wero in the ocean of that era. Mr. Danvin, who made some curious inquiries on this point, fnrther suggested that the matter mii.Tht have intermediately passed through the bo dies of worms and fi::3h, such as feed on the corals of the present day, and in whose stornachs he has found ir_npure chalk. This, ho\vever, cannot be 3 full explanatwn of the production of chalk, if we admit some more recr.~t discovel'ie of Professor Ehrenberg. That master of miCl'oscopic investigation announces, that chalk. is .composed partly of " i:1organic particles of irregular ell1 pt1eal st. rue· turc and granular slaty dispo!:)ition,, and partly of shell3 oi' inconceivable minuteness, "varying from the onetwelfth to the two hundred and eighty-eighth part of a CRETA CEo as FORJ\tA TION. G3 ln:e,.'-a c?bic inch of the substance containing above ten m~llwns of tl:e1n! The chalk of the north of Em·ope contains, he says, a larger proportwn of the inorO'anic 1natter · th~t o1 th? .so~th, a.larger proportion of. the 0organic mat~ ~ei, be1 ng 1 n some Instances almost entuely composed of 1t. H~ has bee_n able. to classify many of these creaturP~ some o.t them be1ng all1ed to the nautilli numuli cyprides &c. T~e shells of some are calcareou's, of oth~rs silice: ous. lVL ~hrenberg· has likewise detected microscopic sea-plants 111 the chalk. The dis~inctivc feature of the uppermost chalk beds in England,. Is the p_rcsence of fiint nodules. These are genera~ ly disposed In layers parallel to each other. It was 1:Gad1ly presumed . by geologists that these masses were fo~~ed by a ch~miciJl atsgr~gation of particles of silica, ongtnally heh~ I.n s~lutwn In the mass of the chalk. But whence the sJliCa H1 a substance so different fi·om it! Ehrer:berg suggest.s that it is composed of the siliceous covenngs of a.portwn ~f the microscopic creatures, whose shell~ ?e has 1n other Instances detected in their original conditiO~. It remarkable that the chalk with -flint abounds 111. th~ north of Europe; that 'Without flints in the south; ~ h1le In t~1e northern chalk, siliceous animalcules aFe wanting~ and 11! the southern present in great quantities. Th~ ~oncluswn seems but natural, that in the one cai:le t~e sthccous cxuvire have been left in their oriO'inal form; In the ot~er ~issolved chemically, and aggregat~d on t~e common pnnc1ple of chemical affinitv into nodules ot fl~nt, probably .concentra~ing, in every i"nstance, upon a p1~ce of decaying organic matter, as has been the case With the nodules of ironstone in the earlier rocks and the spherules of the ool.i.te. ' What is more remarkable, M. Ehrenberg has ascertained that a\least flfty-s~ven. specie~ of the microscopic animals of tl.e ch~lk, bemg 1nfusona ahd calcareous-shelle~ polythalamia, are still found living in various parts of the ea~·th. These species are the most abundant in the rock. S1~gly they are the most uuimportant of all animals, but 1n the mass, forming as they do such enormous ~trata over a large part of the earth's surface, they have an Importance greatly ~xeeedi n.g that of the largest and noblest of the b_eas!s of the field. Moreover, these species have a pecul~ar Interest, as the only specific types of that earl1 age whiCh are reproduced in the present day. Spe- |
