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scenes that once were here, perhaps long before the human epoch, can we find around or beneath us? There are no open sections in the soil around us but such as are overgrown with grass.
But here, hard by, is a brick-field. We rejoice at the sight, and make our way to the spot.
Let all brick-fields be prized by the geological excursionist, especially in a district remote from the sea coast, and deficient in sections such as cliffs and chalk-pits afford. To this particular spot has been brought a vast mass of clay which has been excavated from the adjacent Finsbury Park, where a large reservoir, about twenty-five feet deep, has recently been dug. This clay will lie here throughout the winter months, for it has to be exposed to the disintegrating effects of the atmosphere before it will be fit for brick-making. The frosts will break up and crumble the refractory lumps. The mass will then be wholly turned over, examined, and tempered for the mill. In these last processes the imbedded fossil remains may be found, for with thumb and finger the clay is carefully examined, and the “race ” taken out, lest it should crack and
" flaw the brick in the baking. “Have many fossils been found here ? ” we inquire. 'Well, yes, at one time and another, Mr.
of Stoke Newington, has had a good many.” “Have any been found lately?” “Yes; a good many sharks' teeth, about twenty feet down, in digging for the reservoir in the Park last year.” Eventually we obtain from our informant a sound and perfect shark's tooth. We select it from a handful of fossil teeth of various species, which he has taken out of the clay. The point of this particular shark's tooth is, even now, dangerously sharp to handle. (The hands of incautious labourers who manipulate clay are often badly cut by these teeth.) The whole surface is smooth and brilliant, and in perfect condition for voracious exercise, much as it was when
“ The ravin'd salt-sea shark"
sought its prey in the superincumbent water of the spot where now we are standing. It is about an inch and a quarter in length from the base to the point. To make quite sure of its authenticity, the practical member of our party consults a private geological manual, and finds therein that the specimen obtained is figured as otodus
obliquus. Didymus is appeased, but we will not complain of a cautious habit of the mind towards discoveries such as this.
Sharks' teeth dug up from the solid ground-embedded deep in a soil which never before has been disturbed by the hands of man ! How came they here, these tokens of a monster whose natural home is the sea ? The enquiry is natural, especially to those who have not yet realised the vast revolutions in physical geography that our earth has undergone, and that the ground all round us in these Middlesex landscapes has shared. But our previous excursions on the Saturday afternoon have prepared us to meet with the phenomenon before us. Let us recall for a moment the lessons of our first geological excursion.
In our Hornsey ramble in the early Spring, we satisfied ourselves by personal observation that this soil which is called the London Clay is neither more nor less than an ancient sea-bottom. The waters were gradually drained off as the land was slowly upheaved. We learned that this vast body of earth is really the vestige of a huge deposit of sediment from a pre-historic sea. We verified to our own satisfaction the character and history which we had found attributed to the London Clay by professional inquirers who had devoted their lives to the subject. If, in the absence of anything so precise as a Mercator's projection, the configuration of the land of this early period could hardly be ascertained, some important contribution to ancient physical geography and zoology around London had been made. We were taken back to the time when nothing in the form of the present configuration of land and distribution of water had begun to exist. This upper floor of the beneath us which we call the London Clay was then gradually being laid upon the yet lower floor of the chalk. On this underlying floor of the chalk-itself an old sea-bottom with great cavities or basins hollowed into its surface—the waters of the newer sea of the London Clay began to pour themselves until they gradually filled up with their darker sediments the great basin beneath us. Further, the original connexion of the London basin with those of Hampshire and Paris has been satisfactorily established, and their subsequent severance has been traced and explained.
“The uniformity of character which prevails in the base of the tertiary formations throughout large areas of the Paris, London, and Hampshire basins, indicates a widely-spread and continuous sea.
After a time, this uniformity of mineral deposit ceases, and
“the variety of the synchronous strata in the different basins proves the increasing disruptions and severance in the original connexion.”
Not only has the extent and range of this old sea been ascertained; its depth, as we saw, has also been satisfactorily gauged. As we, on our way to Hornsey and Highgate, are now walking on the bed of a sea which has been gradually slanted off into lower levels, we may even know how high above us was the level of the superincumbent waters. How is this ? Simply because, as we are told by biologists, tribes of creatures which inhabit the sea exist only at zones of depth that are suited to their species, as creatures upon the land live only in certain zones of altitude.
“On the authority of Professor Rupert Jones, the indication of sea-depth for the London Clay afforded by the foraminifera is, that the water was about 100 fathoms deep” (600 feet).
Let us remember this as we are safely walking on the floor of this ancient sea at Hornsey, uncumbered with diving gear, glass headpieces, and other Polytechnic apparatus.
* On the Area of the Tertiary Sea. probably in part prolonged until the - It is probable that the Tertiary completion of the Isle of Wight series Sea at first extended uninterruptedly -the London district in the meanover the London, Hampshire, and time assuming a more isolated posiParis areas—that at a period coeval tion, and diverging sooner from be. with the change of condition both in neath the sea.—PRESTWICH, Jour. structure and fauna which is evident Geol. Soc., vol. ii. at the base of the London and Bog- On the Existence of Shallow-water nor Clays a separation took place Genera in all Depths of the London between the Paris and Hampshire Clay. — The lithological character areas, leaving the latter still con- of the London Clay denotes a trannected with that of London during quil and uniform deposit during some the deposition of the London Clay, length of time; and as the evidence and that after this period the com- of fossils proves that the condition munication between the French and of animal life was similar at the end English tertiaries was restored, as to that which existed at the comevinced by the introduction th mencement of this period, it follows Bracklesham beds of so many French that there must have been throughspecies, so that the connexion was out its duration a quiet and gradual
The shark tribe, of whose dental furniture we have secured a sample by our visit to this brick-field, and the ray tribe, both now :so rare on our coasts, seem to have swarmed in the old sea of the London Clay.
“Few fossils are more common in the London Clay than the teeth of the shark. They retain all their brilliancy and sharpness, and bear the closest resemblance to the teeth of the existing shark. The other parts of the skeleton, owing to their soft texture, are, with the exception of a few bones of the back, rarely met with.”
Some of these old sharks, judging by the remains found at Finsbury Park, the Isle of Sheppey, and elsewhere, must have been formidable creatures. A vertebra in Mr. Bowerbank's collection measures four inches in diameter. The fish to which this belonged was probably not less than from thirty to forty feet in length.
We now resume our route, and slowly ascend the slope of Mount Pleasant by a foot-path perversely named “Cut-throat Lane.” The incident of the shark's tooth revives in our company a conversation upon the general subject of the fauna of the London Clay period -the crocodiles, turtles, great boa-like serpents, and other creatures which once lived and died in this meridian and latitude of ours, leaving to us, their successors, evidence of the different climate which once prevailed in these now temperate regions. But let us not
subsidence of the bed of the sea. Sheppey, of a departure during part The increasing depth caused by the of this period from these uniform subsidence appears to have been con- zoological conditions; but we here stantly neutralized by an accumula- find various species of Cephalopoda, tion of sediment, equal, or nearly so, Echinodermata, some Brachiopoda, and to the amount of depression. By the of a generally deeper sea Testacea. joint and counteracting effects of . . From the operation of more these two causes a nearly uniform numerous zones of depth, adapted to and moderate depth was maintained the existence of more throughout the seas then covering classes of Testacea, results probably these districts, enabling the Panopæa, the more diversified fauna of HampPholadomya, Pinna, and other shallow- stead, Highgate, and Sheppey.water genera, to exist all through this Ibid. vol. iii. geological period, to the exclusion of Although the fossils are alike the more varied fauna which greater throughout this formation, yet the changes of depth would have pro- same species are not found equally duced. But we have indications, as from top to bottom, and by a very we proceed further eastward in the careful comparison of the fossils London district to Highgate and from different places, Mr. Prestwich
neglect to observe the form of the ground as we walk and converse. Before returning to the subject of the former climate of these latitudes, let us acquaint ourselves with the configuration of the ground at this spot. The climate has gone, but the ground remains for our observation.
Mount Pleasant, Hornsey-in some maps called Crouch Hillis part of that remarkable horse-shoe ridge of high ground which forms a crescent on the west, north, and east sides of London. This ridge is popularly known as the Highgate and Hampstead range. It begins westward with the highest ground in Hyde Park. It may then be traced by Paddington, Barrow Hill, Primrose Hill, Hampstead, Caen Wood, Highgate, Hornsey Lane, to Mount Pleasant, where we are standing. Here the ridge almost dies away for about a mile and a half into a depression, through which at some distant period a strong current probably ran, wearing for itself a channel and transporting the surface soil to an estuary. It appears again about half-a-mile west of Cambridge Road, and forms the highest ground of Stamford Hill, Upper Clapton, and Homerton.* It then skirts the valley of the Lea for about three miles, until it falls off into the flats of Bow, Bromley, and Stratford.
The contours of an old world-surface, as exhibited by such a range
has been able to make out the existence of four zones' in this thick
6 mass of clay. He says, “The organic remains of the London Clay are distributed in groups marking particular zones.
Confining ourselves to the London district, it would appear that, although a great proportion cf the fossils range at intervals vertically throughout the London Clay, yet their development is very different in different zones, being abundant in some and scarce in others, whilst each zone is further marked by a few characteristic species, thus forming distinct, although nearly related groups. ... It must not, however, be supposed that the fossils
are dispersed either vertically or hori. zontally with any uniformity as to numbers.
These zones are necessarily artificial, as no actual division exists, and the organic remains and mineral characters are continuous; but they serve to mark the distinct conditions of the fauna and flora at particular periods, and show the prevalence generally of like forms on the same levels; they are not, however, to be taken by any means as constant, but merely as local centres for convenient reference and grouping.'”—W. WHITAKER, B.A., op. cit.
* Roads and Road-making. By E. · Law, C.E.