and Pleurotoma at 2,090 fms. The Terebratula striata of the Chalk is considered very like the T. caput-serpentis dredged at depths varying from 70 to 1,200 fathoms.

On physical grounds both Thomson and Carpenter are inclined to believe that a considerable portion of the deep Atlantic area has been continually under water, and that consequently a deposit has been forming there uninterruptedly, from the period of the Chalk to our own; and that our Tertiary beds represent the mineral accumulations and the fauna of the margin of some sea whose deep-sea fauna is unknown, being still beneath the Atlantic.'

Chloritic Marl.

The Chloritic Marl is a deposit concerning which very different opinions have been expressed, especially in regard to its classification, whether it should be placed with the Chalk or with the Upper Greensand.

It seems (according to Mr. Topley) that in the Wealden district it is more closely allied to the Greensand, and in Cambridgeshire and in the west of England it seems very clearly to constitute the basement bed of the Chalk. Such being the case it may very naturally be regarded as a passage-bed, but, at the same time, it would obviously be inconvenient to designate it as a separate formation owing to its feeble development; and therefore, looking to the general nature of its organic remains and of its lithological characters, it may best be included with the Chalk, as Edward Forbes originally advocated.

The Chloritic Marl consists of white or pale-yellow marl with dark green glauconitic grains, phosphatic nodules, and nodules of iron- pyrites. Sometimes the bed is indurated. The origin of the phosphatic nodules, often miscalled 'Co

1 The great Tertiary marine fauna of the Eastern Nummulitic sea may, however, be regarded as an equivalent.

prolites,' is a subject that has been much discussed. Perhaps the simplest explanation is that given by the Rev. T. G. Bonney; he points out that Phosphate of lime is present in small quantities in the sea, in several rivers, and in numerous mineral springs; it is found in many plant and animal remains; and in the form of Apatite it is met with in many rocks. He considers that the phosphatic nodules are due to concretionary action, and have been formed by segregation out of mud saturated with phosphate of lime. The 'coprolites' in the Chloritic Marl of Cambridge he considers as derived from more extensive deposits of Gault age.

In the Wealden District the Chloritic Marl occurs in the neighbourhood of Alton and Seiborne. It contains a good deal of phosphatic matter, and the beds have been largely worked at Froyle. According to Mr. Bristow the thickness. of the bed varies from a few inches to 10 or 15 feet.

6

The celebrated coprolite' beds found near Farnham in Surrey, and worked at Dippen Hall, vary in thickness from 2 to 15 feet. This neighbourhood is celebrated for its hopgardens.

In the Isle of Wight the Chloritic Marl is about 5 feet in thickness, and contains phosphatic nodules.

In Dorsetshire the Chloritic Marl has a thickness of about 18 inches, and contains phosphatic nodules. It is well seen in the neighbourhood of Chard, where it is exceedingly fossiliferous. (See fig. 18.)

At

In Wiltshire the thickness is sometimes 6 feet. Wroughton, near Swindon, the phosphatic bed is 18 inches. thick.

Between Hitchin and Cambridge the Coprolite' beds have been largely worked.

The so-called Cambridge Upper Greensand, which consists merely of a bed about a foot in thickness containing Phosphatic nodules and many derived fossils, Inoceramus,

Terebratula, &c., is now regarded as belonging to the Chalk.

Mr. Jukes-Browne observes that the coprolites,' fossils, and green grains which it contains have been mainly derived from the denudation of the Upper Gault; and he considers the bed to belong to the base of the Chalk Marl, the Upper Greensand being absent. As Mr. Bonney observes, it is probably homotaxeous with the Chloritic Marl.

This phosphatic deposit is particularly rich in vertebrate remains, including one Bird of the Ornithosaurian group, many Reptiles, including Ichthyosaurus, Plesiosaurus, Chelone, and the Deinosaurian Acanthopholis (also found in the Chalk Marl of Folkestone); likewise many Fish-remains.

Range of the Chalk.

The Chalk extends from Flamborough Head inland, forming the Yorkshire Wolds, and thence running beneath the Humber near Hull to form the Lincolnshire Wolds.

It constitutes the foundation of the greater part of Norfolk and Suffolk, but is in these counties very much concealed by Glacial Drifts, and does not there appear in such conspicuous hills as those which extend from Royston in a south-westerly direction, forming the Royston and Luton Downs, the Chiltern Hills, the Marlborough Downs, and Salisbury Plain. Thence the Chalk stretches out irregularly to the west beyond Dorchester, and is found in outliers near Chard, Seaton, and Sidmouth. Eastwards of Salisbury Plain the Chalk forms a large extent of Hampshire, it is found in the Isle of Wight, and borders the Wealden district, forming the cliffs from Margate to Folkestone on the north, and those from Beachy Head to Brighton and Little Hampton on the south. It is also exposed at Gravesend and Grays Thurrock.

In Yorkshire the following succession of the Chalk strata has been determined by Professor Phillips :

Chalk

Upper Chalk, rich in Spongiade, Marsupites, &c.
Middle Chalk with flints, and few fossils

Red and Grey Chalk, with many fossils. Red Chalk, Belemnites Listeri, &c.

Feet

100

. 400

100

40

30

He observes that the unconformability of the Chalk to all the strata beneath it is a striking feature of the geology of Yorkshire. The lowest bed of Red Chalk is the equivalent of the Hunstanton Limestone.

Mr. Judd has pointed out in regard to the Chalk of Lincolnshire, that the occurrence of the flint layers is very irregular. Above the Chalk Marl is a considerable thickness of hard Chalk (as in Norfolk) which has been used for buildingpurposes. Louth Abbey was in great part constructed of it. Near Louth this series contains a well marked bed of Red Chalk, 5 or 6 feet in thickness, and some other bands of a pinkish hue, all distinct from and much above the Hunstan

ton limestone. The 'Sponge-bed' with Spongia paradoxica is found at the base of the Chalk series, graduating into the Hunstanton limestone. The Wolds of Lincolnshire are much more covered with superficial deposits than the downs of the south of England, or even Norfolk; hence the district of the Chalk in this county does not present that uniformly bare and arid appearance so characteristic of it in most parts; in fact, nearly the whole of it has now been brought under the plough, and with the most satisfactory results.

In Norfolk the Chalk was divided by Mr. Samuel Wood

ward into:

Upper Chalk, with flints.

Medial Chalk, with few flints.

Lower or Hard Chalk.

Chalk Marl.

The thickness of the Chalk-with-flints (Upper and Medial) was proved to be 1,050 feet in a well-boring at Messrs. Colman's at Norwich; and that of the Chalk-without-flints (including the Hard Chalk) 102 feet. The Chalk Marl is only about 3 feet in thickness, and it is in a bed of white Chalk occurring at the base of it at Hunstanton that the Spongia paradoxica is found: this bed, called the Sponge Bed, is scarcely 18 inches in thickness, and may represent the Upper Greensand.' (See p. 238.)

The Hard Chalk has been much used for building-purposes. It has been largely quarried at Stoke Ferry and Whittington; it has yielded Ichthyosaurus campylodon; also Ammonites peramplus, and A. Austenii, each about 2 feet in diameter.

The Upper Chalk (very rarely the Medial Chalk) is characterized by the presence of the gigantic flints termed paramoudras or potstones, often 3 feet in length and 1 foot in

1 Mr. Rose has estimated the maximum thickness of the Chalk at Norwich at 1,192 feet.