GREEN SAND AND CHALK MARL.

273

extensive tract of sand universally traceable beneath the escarpment and inferior termination of the chalk ranges. This series is exhibited in the clearest manner in the southern counties of England ;-the middle and northern have not been so well explored. The sandy beds are divided into two groups, with an interjacent stratum of clay. The upper sandstone is interspersed with numerous specks of a greenish substance; the lower has a deep ferruginous hue, from brown oxide of iron. Hence the

first is called the green sand; the second the iron sand formation. The upper bed is parted from the chalk by argillo-calcareous strata, but their line of division is not very well marked on account of the graduation of the lower chalk into them. mixed beds have been called chalk marl.

These

Reck

oning from below in the order of superposition, we

have therefore,

1. Iron sand.

2. Interjacent clay, which being found extensively in the Wealds of Kent, Surrey, and Sussex, is called Weald clay.

3. Green sand.

4. Chalk marl.

All these strata are probably of marine origin. Formations similar to these in England, are found skirting the chalk of Boulogne, at its western limit about Honfleur and Havre, and its eastern boundary at Valenciennes; where the green sand assumes a conglomerate appearance, and is called turtia. Analogous formations have been observed in Switzerland. It is probable that the sandstone of Saxony, called Quadersandstein, belongs to the same series.

Y

In other parts of the world, we possess no means of tracing these beds.

1. Iron or Hastings Sand.-This formation may be studied with most advantage in the neighbourhood of Hastings. It consists of a series of strata in which sand and sandstone prevail, alternating occasionally with subordinate beds of clay, marl, fuller's earth, and ochre. The sand is entirely siliceous, and contains iron in such quantity as formerly to have rendered it worth working as an ore of that metal. The sandstones often form coarse grained conglomerates, which consist of pebbles from the size of a pigeon's egg to a pin's head, imbedded in a ferruginous cement. Thence a regular gradation may be traced to a very fine grained sandstone, affording flags for pavements and for building stones. Ferns, charred wood, and other supposed associates of coal, occur in the white and gray sandstones of this series, but rarely in the ferruginous. The beds of fuller's earth which occasionally alternate here, have been extensively worked in Bedfordshire. The ochre of Shotover hill occupies a similar geological position.

The organic remains of this bed have been imperfectly explored. They are not numerous; but the nautilus, belemnites, ammonites, ostrea, terebratula, and spines of an echinus cidaris have been found. The most abundant fossils are the spongitæ, of which many varieties, tubular, funnel-shaped, and palmated occur. The richest locality is at Farringdon in Berkshire. Beautiful minute corallines have

been also observed.

In the west of Cambridgeshire this formation is

ABUNDANT ORGANIC REMAINS OF GREEN SAND. 275

well exhibited, and it may be traced thence throughout the southern part of the island. In the midland counties it constitutes the mass of a chain of hills extending through Bedfordshire, and forms the summits of the same chain which ranges through Bucks, Oxon, Berks and Wilts. Its greatest thickness appears to lie in the Weald country, where it amounts to 500 feet. In inclination, its strata conform to those of the chalk which lies over it. It constitutes in many places a fertile soil, and is favourable to the growth of wood. Like all other loose and porous mineral masses, which are divided by a few tenacious layers, this formation can furnish water only at such partitions; and hence the wells are here deep, and from the abundance of iron they are chalybeate. Tunbridge Wells is a notable instance.

2. Weald Clay.This is exhibited on the largest scale in the Wealds of Kent, Surrey, and Sussex, where it separates the central nucleus of ferruginous sand from the encircling ranges of the green sand strata. This bed varies in quality from a dark tenacious clay, to a blue or gray calcareous marl of an earthy and friable texture. Layers of argillo-calcareous concretions occur replete with shells of the genus Vivipara fluviorum. Their interior is filled with calcareous spar; which when cut and polished is called Petworth marble. Its organic remains have not been enumerated; nor can we infer any thing as to the formation from the above single shell. In the Weald of Kent the thickness of the bed is about 300 feet; in the Isle of Wight only 100.

3. Green sand.-This formation is one of the

most important between the oolites and chalk, both from its mass and the number and beauty of its organic remains. In the southern counties of England the bed is thick and easily traced; but it is more obscure in the midland and northern. The green sand is siliceous, and is sometimes consolidated into sandstone with a calcareous cement. The nature of the green earthy colouring matter is not well known. It is probably some form of iron, along with micaceous particles. There are subordinate beds and masses of chert, and veins of chalcedony; with alternating beds and nodules of limestone called Rag in the Isle of Wight. It is the Kentish rag. This limestone is perfectly distinct from that of Portland in geological position, character, and fossils. There are occasional parting seams of clay. Iron pyrites occurs in this rock at Folkstone, and hematitic iron in the ferruginous beds. Crystallized sulphate of barytes of a yellow colour, with its interstices filled with opaque quartz, has been observed.

Organic Remains.-These are extremely numerous. At Blackdown, and other places, they occur in a state of beautiful preservation, imbedded in the siliceous varieties of the rock, the original calcareous matter of the fossil being entirely supplanted by chalcedonic infiltration. The quarries of Blackdown afford 150 species of testacea. Under the family Echinus we have the divisions cidaris and spatangus, and one small species of conalus. This formation is the lowest in which the spatangi have hitherto been found in England; and the only one besides the chalk, that seems to contain the

NATURE OF CHALK MARL AND EXTENT.

277

conalus. The encrinites are few and unimportant; as well as the coralloids. The alcyonic remains are however both more numerous and important in this formation than in any other except except the chalk. We have the ramose; the funnel-shaped (figured in the frontispiece to Parkinson's Organic Remains); the tulip shaped; one with a large head divided into many lobes, standing on a short neck; one shaped like a cucumber. Silicified wood also occurs.

This formation may be traced very extensively beneath the chalk. In Wiltshire it constitutes hills standing in advance of those of the chalk formation, and nearly equalling them in height. On the confines of Dorset and Devon it occurs in high and insulated masses, called outliers. In the course of its progress to the south-west, the green sand successively overlies the terminations of the oolites and the lias; becoming immediately incumbent on the newer red sandstone, in the western part of Blackdown. Through the isles of Purbeck and Wight, it attends the elevated strata of the chalk, with a conformable inclination. Its hills do not exceed 800 or 900 feet. Its thickness is in many places 200 feet. It forms a light loamy soil, of considerable fertility. From its porous nature, it is often necessary to pierce it deeply before we reach the water kept up by the retentive substratum of Weald-clay.

4. Chalk Marl.-This formation consists apparently of three ingredients intimately mixed; 1st, cretaceous matter; 2d, argillaceous matter; 3d, sand. The cretaceous matter is harder than chalk, is laminated, and will not mark like it. Its colour is gray or mottled, and it falls to pieces when wetted