218 



HARDWICKE'S SCIENCE-GOSSIP. 



upheaved, it follows that this denudation has heen 

 so operative as to wear away the softer beds and to 

 leave the harder standing. Hence the physical geo- 

 graphy of the whole formation differs according to 

 the underlying geology. Deep valleys or extensive 

 plains lie where the clayey or argillaceous strata 

 crop out; and broken hills, frequently with more 

 or less steep westerly escarpments, indicate the 

 areas occupied by the limestones and harder sand- 

 stones. 



As might be expected, when it is remembered that 

 this series of deposits was formed chiefly along the 

 old sea-bottoms, there must have been an extensive 

 and long- continued list of geographical changes 

 rung whilst it went on. The bed of the ocean was 

 alternately the receptacle for the fine muds brought 

 down by rivers, along whose deltas grew the rich 

 vegetation locked up in the coal-seams and shales of 

 the Lower Oolite near Scarborough. Then we have 

 evidence of a depression of the area, which removed 

 the sphere of deposition of the mud, and brought 

 clear water over the site. Here the physical condi- 

 tions allowed mollusca, corals, &c, to swarm in 

 abundance, and their accumulated remains thus 

 formed the limestones. , Calcareous sandstones 

 were formed out of the comminuted coral reefs, 

 shells, &c. Occasionally, influxes of mud killed off 

 large numbers of encrinites, as at Bradford, near 

 Bath, and buried them beneath its debacle, clear 

 water returning shortly afterwards, as the parasitic 

 zoophytes, &c, which attached themselves to the 

 broken joints of the encrinites, plainly indicate. At 

 length the deposits more or less filled up the shal- 

 lower parts of the sea, and upheaval converted a 

 portion of it into dry land. The hollows of this 

 land became freshwater lakes, in which swarms of 

 Planorbis, Pahidina, and other well-known fresh- 

 water snails lived. The water was clear, and there 

 was no great amount of muddy materials carried 

 into these lakes. Time only was required for the 

 shells to accumulate along their floors to "such an 

 extent, that, in their solidified condition, they form 

 the bulk of that well-known "Purbeck Marble" of 

 which I am a humble and minute portion. Occa- 

 sionally the sea-waters backed up the fresh, and 

 encroached on some portion of the lakes, holding 

 the place sufficiently long for brackish-water shells 

 to live and multiply there, and to leave their remains 

 behind them in token of what I have said. Even 

 the pure sea-water once or twice gained ground, as 

 the beds of fossil oysters, &c. intercalated in the 

 Purbeck beds reasonably show us. In these 

 different beds you find evidences of nearly all kinds 

 of deposition, from the tolerably deep water in 

 which the " Coral Rag " was formed, chiefly as a 

 coral reef, to the ripple-marked flagstones of the 

 " Great Oolite," in which also you get tracks of 

 worms, crustaceans, &c. The total thickness of the 

 entire series is about [two thousand four hundred 



feet, which alone will give you some idea of the 

 enormous period of time represented by them. 



There are few geological formations so rich in 

 fossils as the Oolite. Not only in individuals, but 

 also in species, the rocks are one vast museum, 

 illustrating a particular stage in the world's past 

 history. You may catch glimpses of life in every 

 form of its enjoyment — in the mighty Saurians 

 which frequented the open seas ; in the busy coral 

 reefs secreting lime; in the bony-plated fishes, 

 whose glistening enamelled scales glanced through 

 the waters. You see the low tide fringed by a 

 vegetation, partly growing on the mud-banks as a 

 swamp, and you distinguish forms now regarded as 

 sub-tropical to Britain. The sea-bed is literally alive 

 with cidaris, bivalves, univalves, sea-lilics, and lamp- 

 shells. Overhead, over land aud water, the flying 

 lizards {Vterodactyles) whirl and swoop. The tiny 

 kangaroo rats and opossums are busy in the forests, 

 some lying in wait for their numerous insect prey, 

 and others, more bloody-minded, are cannibally 

 inclined ! The great freshwater lakes, along whose 

 floors I was formed by the simple accumulation 

 of ordinary freshwater shells, were set in a dense 

 and beautiful framework of pine-trees, of cycads, 

 zamias, and tree-ferns. But, vast as the period of 

 time is since this, the last of the oolitic series, was 

 formed, numbering, as it undoubtedly does, millions 

 of years, it has all elapsed within the lifetime of 

 existing genera of shells ! The Paludina, which 

 principally make up my bulk, can hardly be told, 

 even by experienced conchologists, from the ordi- 

 nary freshwater snails which still inhabit English 

 rivers ! In structure of limb, tooth, and general 

 adaptation, the highest orders of animals then 

 existing were wonderfully like their Australian 

 and North- American brethren. 



In the swampier places, at the beginning of the 

 oolitic period, where the vegetation grew thick and 

 rank, beds of peat were formed and covered up by 

 mud. This peat subsequently became coal. The 

 iron diffused through the muddy mass was influenced 

 by chemical action, so as to reunite and segregate, 

 as an argillaceous carbonate, into layers and nodules 

 of iron-stone. In this respect, the physical condi- 

 tions greatly resembled those which existed during 

 the Carboniferous epoch, and therefore the results 

 are very similar. All you have to do is to transpose 

 the animals and plants of the two eras, the differ- 

 ence in each of which represents the amount of 

 time which had elapsed between them, and in 

 which the vital modifications had taken place. In 

 the Stonesfield slate— a calcareous shak, and a 

 capital burial-ground of extinct animals—there 

 were entombed the remains of at least four species 

 of mammalia. As I before remarked, however, 

 all the warm-blooded animals which lived during 

 the Oolitic period belonged to the lowest order of 

 their kind— the marsupials, or pouched animals, 



