272 



HARDWICKE'S SCIENCE-GOSSIP. 



and step by step migrated southerly. One species, 

 doubtless the liueal descendant of those which lived 

 over what is now called Suffolk, is met with in the 

 TV r est Indian seas. Most of the "southerly" shells' 

 however, are to be found in the Mediterranean. 



By the slow accumulation of dead shells, corals, 

 &c, cemented by the smaller tests of foraminifera, 

 the Coralline Crag eventually attained a thickness 

 of fifty feet. It was slowly upheaved, aud subjected 

 to great erosion by the action of marine currents 

 which scooped out great hollows. When the up! 

 ward movement was arrested for a time, in these 

 hollows was thrown down another and later series 

 of deposits, termed the "Red Crag." This Crag, 

 whose prevailing colour gives to it its name, has a 

 much wider extension than the older member of 

 our series. Just before it was formed, the same 

 wear- and -tear which had so effectually cut clown 

 the Coralline Crag also denuded the underlying 

 London Clay. For ages before;;the depression took 

 place which brought the Crag seas over Suffolk, 

 this had been a land surface, over which had roamed 

 hosts of wild and extinct animals. The wear-and- 

 tear had loosened and washed out the fossils of the 

 London Clay, so that underneath the Red Crag, and 

 with the latter resting on it, you find a bed of 

 stones in which are huge teeth of sharks, bones of 

 whales, teeth of tapir, elephant, mastodon, &c. 

 The stones are those so-called " coprolites " which 

 make the Red Crag so valuable. These are nothing 

 more or less than phosphatic nodules in a re-depo- 

 sited state. 



In this Red Crag sea there lived over two hun- 

 dred and fifty species of shell-fish, among which, 

 however, you will only find about thirteen of the 

 "Southern" forms. The "Northern" types are 

 also on the increase, so that you have in these two 

 facts an indication of an increased rigour of climate. 

 The sea was not so deep as during the formation of 

 the older crag, so that you get a great many more 

 shallow-water shells, among which those of the 

 Limpet family are most abundant. The small single 

 corals were very numerous in places, and the little 

 cowrie-shells literally swarmed everywhere. That 

 the water was shallow you may see for yourself 

 whenever you visit a Red Crag pit, for you cannot 

 fail to be struck by the lines of false current bed- 

 ding which everywhere meet your eye. The rough 

 marine action testified to by these phenomena 

 ground up the more delicate shells into the bran- 

 like appearance of which the matrix of the crag is 

 composed. 



Extending in a north-easterly direction, towards 

 the conclusion of the Red Crag era, and when its 

 beds had been formed to a depth of at least twenty 

 feet, was a shallow -estuary, which ran sinuously 

 through the bare chalk into what is now Norfolk. 

 It occupied the very site of the city of Norwich, 

 and reached its head about four miles beyond, 



where a small river poured its waters into it, so as 

 to produce brackish water conditions. You will 

 see, therefore, that this later, or " Norwich Crag," 

 as it is usually called, was merely a fluvio-marine 

 extension of the more purely marine Red Crag. 

 Owing to its being formed under different condi- 

 tions, the fossils of the Norwich Crag differ very 

 much from those of its older brethren. You meet 

 with no corals or other shells which indicate toler- 

 ably deep water. Instead you have abundance 

 of periwinkles, cockles, mussels, whelks, purple 

 shells, &c., associated with myriads of Tellina aud 

 Mactra, as well as winkle-traps and Ceritliiion. 

 Associated with these are brackish-water shells, 

 and such purely fresh-water mollusca as Lijmnea, 

 Planorbis, &c, and even land-snails, which had 

 been brought down by the tributary streams, 

 aud eventually strewn along the bottom of 

 the estuary where the Norwich Crag was slowly 

 forming. Altogether, no fewer than one hundred 

 and twenty species of mollusca have been derived 

 from this bed. Underneath it you may see a similar 

 stone bed to that underlying the Red Crag in 

 Suffolk, and, like it, testifying to its having been 

 an old land-surface of the solid chalk ; for here 

 are abundant remains of deer, elephant, rhinoceros, 

 mastodon, &c. 



Such are the relative geological conditions of us 

 three Crags. After the formation of the latter, a 

 depression ensued, which brought the sea over what 

 had previously been merely an estuary, and along 

 its floor was formed another bed of crag, in which 

 marine shells only have been met with. At Aldeby, 

 on the borders of Suffolk, you may see the shells of 

 this bed occupying their original position, the Myas, 

 for example, being found erect in the sand. Neither 

 in the old Norwich Crag, nor in this later bed, do 

 you come across any " southern " shells ; whilst it 

 is evident that the percentage of "northern" 

 species was proportionately increasing. This is 

 good evidence of the fast-encroaching cold, — a cold 

 which shortly afterwards set in, as the drift-beds 

 overlying these crags, and into which the upper- 

 most beds silently pass, plainly attest. The Upper 

 Crag, indeed, is a sort of bracket between the Plio- 

 cene and the Pleistocene, or " Glacial " series. 



It is interesting to note, as you analyze the shells 

 of the crags we have mentioned, how the percent- 

 ages of recent or living shells to those which are 

 extinct bears out their relative ages. Thus you 

 find less than ten per cent, of extinct shells in 

 the Upper Crag just named. In the Norwich beds 

 there are eighteen per cent., in the Red Crag 

 twenty-five, and in the Coralline Crag thirty-one. 

 How long it is since the Norwich Crag was formed, 

 you may gather by the fact that some of its repre- 

 sentative shells are now living only in certain parts 

 of the Pacific ! 



There are beds of the same age as the Coralline 



