28 



NATURAL inSTOKY OF THE PKLMEvfAL WOKLD. 



Mnximiini Tliick- 

 lifbS in l'"eet. 



Gault 150 



Lower greensand, ..... 850 

 Wealden beds, ISUO 



The Maestricht beds consist of pisolitio limestones* 

 in tlie north of France, and of loose yellowish sand- 

 stones in Holland. 



The clialk with flints abounds in the south-east of 

 England, the flints occurring in long parallel lines of 

 remarkable regularity ; so that a section of the rock, 

 viewed from a distance, looks as if it had been scored 

 with the help of a ruler. The limestone is of a pure 

 wliite, pulverulent quality; and as it is usually too soft 

 for building purposes, is chiefly quarried for the lime- 

 kilns. The layers of flints never exceed a few inches 

 in thickness. They are often found in nodules at 

 intervals of from two to four feet. Iron pyrites is 

 frequently met witli in radiated nodules, which readily 

 decompose, and colour the rock with rusty hues, as if 

 from a clialybeate spring. 



Chalk without flints differs from the upper chalk only 

 in the absence of the flints. 



Chalk marl is the white chalk indurated by a gra- 

 dual admixture of argillaceous matter, and transformed, 

 as it were, into a " pale, bufif-coloureil marl," or argil- 

 laceous limestone, generally of sufilcient firmness and 

 hardness for the builder's use. 



The upper greensand is composed of alternating 

 layers of sands, clays, and limestones, frequently 

 coloured green, as its name indicates, by the presence 

 of a chlovitic mineral. 



Gault is a dark tenacious clay used for brickmaking. 

 lis occtu'renco between the strata of the chalk is a 

 cause of the numerous landslips that occur on the 

 southern coast and in the Isle of Wight. The action 

 of subterraneous springs reduces it to a soft consistent 

 state, and it then oozes gradually through the clialk, 

 whose upper strata are consequently precipitated for- 

 ward. It is in this way the wild romantic region of 

 the Undercliff, in the Isle of Wight, was formed. 



The lower greensand resembles the upper in every 

 particular. 



The Wealden, which in England occupies a district of 

 -Kent and Sussex long known as the Weald, is divided 

 into two groups, the Wealden clay and the Hastings 

 sand. It consists of a series of shales and sandstones, 

 in which some highly interesting fossil remains have 

 been discovered. 



From these brief remarks it will be seen that the 

 characteristic feature of tlie Cretaceous formation is 

 the chalk {crda), whence it derives its name. This 

 chalk is a white, soft, and friable or pulverulent sub- 

 stance, almost wholly composed of limestone; the only 

 foreign matter which occurs in any abundance being 

 the silex, which forms its nodules or regular layers of 

 flint. The question now arises. Of what does this 

 chalk or limestone consist ? Hitherto our remarks will 

 have been of interest only to the geologist; but the 

 question we have just proposed concerns the naturalist. 

 For limestone consists of comminuted shells, such as 



* Pisolitic, from pisiim, a pea, in reference to the size of tlje 

 conipoucr.t parts. 



Foraminifera, mixed with tlie disintegrated prisms of 

 larger shells, PinnK, Cytlicrinai, Diatomacea;,and Ento- 

 mostracca;. 



It is a difficult problem, says a distinguished geolo- 

 gist, to account for the source of the enormous masses 

 of carbonate of lime that compose nearly one-eighth 

 part of the superficial crust of tlie globe. Some have 

 referred it entirely to the secretions of marine animais, 

 and to such an origin we must obviously assign the 

 Cretaceous strata composed of comminuted shells and 

 corallines ; but until it can be shown that these animals 

 had the power of forming lime from other elements, wo 

 must suppose that they derived it from the sea, either 

 directly or through the medium of plants. 



In either case it remains for us to discover the 

 source whence the sea obtained, not only these supplies 

 of carbonate of lime for its animal iidiabitants, but also 

 the still larger quantities of the same substance that 

 have been precipitated in the form of calcareous strata. 

 It is impossible to suppose that it resulted, like sands 

 and days, from the mechanical detritus of the granitic 

 rocks, because the quantity of lime which they contain 

 bears no proportion to its large amount among the 

 derivative rocks. The onl}' remaining hypothesis seems 

 to be " that lime was continually introduced to lakes 

 and seas by water that had percolated rocks through 

 which calcareous earth was disseminated." 



The origin of these large quantities of silex wdiich 

 constitute the chert and (lint beds of stratified forma- 

 tions. Dr. Buckland refers to the waters of hot springs, 

 holding silicious earth in solution, and do|)Ositing it on 

 exposure to reduced degrees of temperature and pres- 

 sure, as silex is deposited by the hot waters of the 

 Iceland geysers. 



From these particulars the reader will infer lliat the 

 Cretaceous formation abounds in fossils. Vegetable 

 remains, indeed, are seldom met with, except in the 

 Wealden beds, but all the divisions of the animal 

 kingdom, except the warm-blooded vertebrata, occur 

 in profusion. The marine fauna presents a copious 

 abundance of Sponges {Spongia, Siplionia, and the like) 

 converted into flints; whence Dr. Bowerbank suggests 

 that their " mission " in the primeval world was to 

 cause the deposit of silicious matter from tlie ocean 

 waters, just as the corals induce the consolidation of 

 its calcareous constituents. Foraminifera (as the Tex- 

 tuUiria, Dulima, and Dentalina) must have literally 

 swarmed ; the greater portion of the Cretaceous strata, 

 as we have already remarked, consisting of their exuvia;. 

 This period was, in fact, the noonday of foramiuiferous 

 life ; it existed in hundreds of species and genera, 

 which, flourishing through their brief career, afterwards 

 covered the ocean bed for leagues upon leagues with 

 their calcareous cases. 



Corals were still met with, but not so plentifully as 

 in the Oolitic seas. Radiata increased in number and 

 developed in beautj', and among the most characteristio 

 fossils of the period are the Diadema, the Goniastcr, 

 the Galcritcs, and other genera of Echini, or Sea- 

 urchins. The exquisite lily-like forms of the Encrinites 

 were now on the wane; but annelids, like the tortuous 

 Vcrmiadaria and winding Scrjnda, teemed in the still 

 genial waters. 



