UNIFORMITY OF THE CHALk tract.

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and Poland. Mr. Conybeare says he has never seen a fossil from any foreign chalk-pit to which an analogue might not be produced from the pits of this island. These are important facts. They demonstrate that formations in very distant regions admit of comparison on satisfactory principles; and illustrate the value of organic remains in establishing these comparisons.

Partial chalk tracts occur beyond the precincts of the preceding European area. A remarkable deposit of chalk forms the basis of the great basaltic area in the north-east angle of Ireland. It contains flints, agrees in its organic remains with England, and rests on green sand (called Mulatto). The thickness of that deposit does not exceed 300 feet.

It is remarkable that America, both South and North, seems to be destitute of chalk; Mr. Maclure asserting positively that it does not exist on that continent.

Chalk is well known to have an earthy dull fracture and a meagre touch, but it occasionally concretes into a hard limestone. This compact chalk has been used in building. As this formation is composed throughout of a series of homogeneous beds of a tender earthy limestone, it does not admit of stratiform subdivisions.

But the numerous beds of nodular flints which lie alternately distributed through the greater part of its mass, form one of its most curious and essential features. They are constantly present in all the upper portions of the formation, but are frequently absent in the lower; affording a criterion by which

the two may be distinguished. Hence the meaning of the terms upper and lower chalk. Flints freshly taken from their native bed, exhibit in their fracture an appearance of moisture. They have indeed been produced most probably by the infiltration of siliceous water through the particles of silica originally deposited among the chalk, some of which are still to be discerned. We can see chalcedonies now forming by the percolation of water through siliceous substances. The presence of a sponge or alcyonium seems to have been peculiarly favourable to the formation of chalcedony in chalk. Wherever chalcedony occurs in flint, a careful examination will detect traces of these zoophytes. Beautiful figures are often produced by their ramifications, radiating through the chalcedony; appearances which become more visible when this hydrophanus stone is plunged in water.

Iron pyrites is the only mineral substance common to all the chalks, and it is found in most, if not all the beds, where it varies in size from a pea to an orange. The nodules are crystalline, with a fibrous and diverging fracture. Enormous blocks of carbonate of lime occasionally occur.

Organic Remains. The individual exuviæ though numerous, belong to a few genera; and probably not a single species will be found, identical in all its characters with any now known to exist. Among the remains of vertebral fish, we may specify teeth of a species of shark, perhaps akin to the squalus galeus, two varieties of the grinding palatal bones belonging to unknown genera, with vertebræ, and scales. Of multilocular

ECHINITE FAMILY CHARACTERS OF CHALK. 285

univalves we have the following genera; ammonites, scaphites, belemnites; but the first rarely occurs in the upper chalk, and the second only in the lower. The varieties are peculiar and characteristic of these beds. There are few spiral univalves in the chalk; though they abound in the newer beds above it. The genera trochus, cirrus, and turbo are mentioned. Serpulæ and spirorbes are not uncommon. Ostrea, pecten, terebratulæ, magas, plagiostoma, dianchora, inoceramus, are genera of bivalves which occur; the four last being extinct. A species of balanus, of the multivalves, has been found. See Plate V.

The echinite family comprehending spatangus, cidaris and ananchytes, may be deemed characteristic of the chalk formation, affording of itself as many shells as all the other testacea do. Many species, and one entire genus are peculiar to it. Thus we have the helmet shaped, the conical, the heartshaped Spatangus Cor anguinum, (see figure on margin;) the spheroidal, the cidaris papillata, cidaris variolata; See Plate V. Of the starfish (asterias) 4 species are

figured by Mr. Parkinson as belonging to the chalk in his beautiful representations of organic remains.

Of the zoophytes, the family encrinus has left several genera in the chalk; pentacrinus; straight encrinus; bottle encrinus; stag's horn encrinus; which are all extinct.

The tortoise encrinus, or marsupite also occurs.

It is known merely as a fossil, and is peculiar to the chalk.

Only one species of the family madrepore has been observed.

Numerous exuviæ have been found of the families Alcyonium (see figure on margin), and Spongia. Their interior substance consists of interwoven fibres, penetrated by pores, formed by reticulations which sometimes run confusedly together, and which at others are regularly decussated. The chalk formation of England stretches with little interruption from Flamborough-head to near Sidmouth, forming a range of hills which are often pretty high, with their precipitous escarpment generally on the north-western side. These cliffs exhibit at their summit the superior soft chalk, replete with horizontal layers of flint: and at their bottom, hard chalk with few flints. Magnificent arches open into the mountain mass of Flamborough-head, on the north, forming vast porticoes to its immense caverns, some of which are exceedingly sombre, while others give a cheerful and romantic passage to the light. In the Yorkshire and Lincolnshire wolds, the chalk tract has an average breadth of rather more than six miles. From Bridlington Bay the chalk proceeds inland, and rises into hills, which have nearly a western direction for about 15 miles, with their escarpment facing the north. These hills then turn southward, and terminate at the Humber. Near the easternmost point of the Lincolnshire coast, the chalk dips

TOPOGRAPHY OF THE ENGLISH CHALK.

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beneath alluvium, whence it re-appears on the northwestern point of Norfolk, adjoining the Wash. From Cambridge, the western limit, it runs southwest, rising at Rayston-downs into a range of hills which continue through Herts, Bedfordshire, and Buckinghamshire to Goring in Oxfordshire; a distance of about 75 miles, with an average breadth of 15 miles. It is there crossed by the valley of the Thames. From the summit of its north-west escarpment, it declines gradually to the south-east, and dips beneath the upper bed of the London basin. On the west of the Thames, the chain is continued to Marlborough-downs, and thence to Bagdon-hill in Wilts. It is thus connected with the cretaceous district of Hampshire, being 50 miles in its longest diameter. Its shortest from north to south, is about 20 miles. This vast area is the central chalk region, from which its branches diverge over the island. The hills above Selbourne form its limits on the east; those skirting the vale of Warminster on the west, and on the north, Inkpen-hill to the south of Salisbury; its highest summit is 1011 feet above the level of the sea. Much of the area thus circumscribed lies in Salisbury plain. Another chain is detached from the northeast angle of the great central mass near Farnham, extending to the Straits of Dover near Folkestone to form the North Downs of Surrey and Kent. This chain bounds the London basin on the south, as the former did on the north. Its escarpment runs from west to east by Guildford, Dorking, Seven Oaks, Maidstone, Folkstone, and the cliffs of Dover. The highest point is at Botley-hill.