Geological facts; or, the crust of the earth, what it is, and what are its uses

by W. G. Barrett

No one can write the history of the Chalk formation, without remembering his indebtedness to the patient and laborious toil of Dr. Mantell in this department, from whose “Geological Excursions round the Isle of Wight,”[^[104]] we give the following extract. “The features of a chalk district are so well known, that a brief notice will render local details unnecessary. The rounded summits of the hills, covered with short verdant turf—the smooth undulated outline of the downs unbroken, save by the sepulchral mounds of the early inhabitants of the country—the coombes and furrows ramifying and extending into the deep valleys, which abruptly terminate at the base of the hills, and appear like dried-up channels of rivulets and streams, though free from all traces of alluvial débris, thus bearing the impress of physical operations, of which the agents that produced them have long since passed away—are phenomena familiar to every one who has travelled over the downs of the south-east of England, and are displayed in the chalk districts of the ‘beautiful isle.’ These features are restricted to the hilly districts of the white chalk, and have resulted from the peculiar nature of the sedimentary detritus,[^[105]] of which the strata comprised in the upper division of the Cretaceous system are composed: for in the lower groups, clays, marls, sands, and sandstones prevail; and where these deposits approach the surface, and form the subsoil, the country is broken and diversified, and the landscape presents a striking contrast with the down scenery, as may be observed in the picturesque district which flanks the escarpment of the chalk hills. It may, perhaps, be necessary to remind the unscientific reader, that these strata are but an insulated portion of an ancient sea-bottom, or in other words, a mass of consolidated sediments formed in the profound depths of the ocean, in a very remote period of the earth’s physical history. This detritus is made up of inorganic and organic materials. The former consist of the débris of the cliffs and shores which encompassed the ancient sea, of the spoils of islands and continents brought down into the ocean by fresh-water currents, and of chemical deposits thrown down from mineral solutions. The organic substances are the durable remains of animals and plants which lived and died in the ocean, and of fluviatile and terrestrial species that were transported from the land by rivers and their tributaries; the whole forming such an assemblage of sedimentary deposits as would probably be presented to observation, if a mass of the bed of the Atlantic ocean, 2,000 feet in thickness, were elevated above the waters, and became dry land.”

We have alluded to the undulating character of the downs, so “well known,” as Mantell says, that “local details are unnecessary.” How correct this is may be seen in the following drawing, which represents a portion of Royston Heath.

All over this heath is found the “Royston crow,” during the winter months. This fine bird migrates hither from Norway, to avoid its severe winters, and is scientifically known as the “Hooded-crow, corvus cornix.” On its first arrival, when it is in its best plumage, it is comparatively tame, allowing the sportsman to approach very near; but as the season advances, acquaintance with the gun makes it very knowing and shy. It associates freely with the other crows, but its nest has never yet been found in England. About March the hooded-crow wholly disappears. The head, throat, and wings are black; the back and breast a “clear smoke-grey.” Norman, the bird-stuffer of this town, has always several fine specimens on hand.

As, in the case of the Carboniferous system, we ventured to say to the reader that it was not all coal, so in the Cretaceous system, we would remind him that it is not all chalk; but without going minutely into the subdivisions which the chalk formation has received, because this unpretending elementary treatise does not profess to teach geology, but simply aims, as we have ventured again and again to repeat, to infuse into the mind a desire of acquaintance with the marvels and truths of this science, we will just indicate the leading divisions and nomenclature of this deposit. First, there is the green sand; that is, first, beginning at the bottom or lower part of the formation: this may be well seen and studied in the neighbourhood of Cambridge, where we have procured many of its characteristic fossils, including several vertebræ and teeth of the otodus, a fish allied to the shark family, such as are figured in the opposite diagram.

FOSSIL TEETH OF FISHES:
FROM UPPER GREEN SAND, CAMBRIDGE.
1. OTODUS.
2. CARCHARIAS.
3. CORAX.
4. OXYRHINA.
5. NOTIDANUS.
6. LAMNA.
7. PTYCHODUS.

FOSSILS FROM THE GAULT, FOLKSTONE.
1. AMMONITE DENTATUS.
2. AM. LAUTUS.
3. AM. SPLENDENS.
4. AM. CRISTATUS.
5. AM. DENARIUS.
6. CATILLUS SULCATUS.

At Potton and Gamlingay in Bedfordshire, and in the neighbourhood, this green sand is highly ferruginous, and the roads and fields present that peculiarly dark-red colour which is first singular and then wearisome to the eye. In the case of the Potton beds, the red colour is caused by oxidization or rust of iron; in the neighbourhood of Cambridge, &c., where there is the green sand, this is owing to the influence of “chloritous silicate of iron.” Then we have the galt or gault, a local term of which we cannot trace the etymology. The gault, however, is not of great thickness, but is likely to be the most interesting department of the Chalk to the beginner, on account of the abundance and peculiar appearance of its fossils. A ramble under the cliffs at Folkstone,[^[106]] where the gault may be seen in perfection, will amply repay any one for toil, dirt, and a few slips and bruises. He will there find evidences of a prolific and prodigal bestowment of life in the innumerable fragments of organic remains every where observable; and if he be patient,—if he won’t go running on from spot to spot, saying, as some do, “Oh, there’s nothing here;” if he will just persevere in a minute examination of every spot where organic remains may be detected, he will not come away without his reward in ammonites, hamites, and other cephalopodous mollusks, and most of them with that peculiar nacreous or mother-of-pearl lustre upon them which renders the fossils of this period so beautiful and attractive. Only we caution the explorer not to buy of the so-called guides. At Dover and Folkstone the rogues have a knack of getting a lump of gault, and sticking into it one or two common pyrites, which are very abundant in the cliff, bits of shell, ammonites, &c.; they then offer this conglomerate for sale, all rounded and smooth, assuring you upon their “sacred honour,” the honour of men who always draw upon their imagination for their facts, that they would not ask so much for it, only on account of its excessive rarity. As good economists always avoid cheap houses, and go to the best shops, so let the young geologist always go to the best shop: let him go to the cliff with his hammer, and work for himself. We picture a few fossils from the gault, only regretting that it is out of the power of our artist to convey their lustrous colours, as well as their curious forms.