land areas of that period to have had a similar trend, at any rate in the northern hemisphere, and to have spread from east to west rather than from north to south like our present continents. It is probable, therefore, as Professor Prestwich suggested,' that a great northern continent stretched across what is now the shallower part of the North Atlantic Ocean, between Norway and Greenland, forming a barrier which excluded the colder waters of the Arctic seas, and allowed the Cretaceous ocean to have the full benefit of the warmer currents from the south.

1 Pres. Address to the Geol. Soc., 1871; "Quart. Journ. Geol. Soc.," vol. xxvii. p. lxviii.

H

CHAPTER XI.

HANTONIAN PERIOD.

AVING elsewhere stated my conviction that the Lyellian divisions of Tertiary time cannot be regarded as systems or groups of equivalent geological value to those which are recognized as divisions of Secondary time,' I need only here explain that no more than two such Tertiary systems can be admitted; for the first of them, including the Eocene and Oligocene series, the name Hantonian has been proposed, and for the second, which includes the Miocene, Pliocene, and Pleistocene series, the name Icenian has been suggested. The same nomenclature will be used in the present volume; and it may be observed that the importance of the break between the Oligocene and the Miocene has recently been recognized by M. de Lapparent."

The simplest division of the British Eocene series into groups of fairly equivalent value is as follows':

Upper

(Hordwell Sands and Barton Clay.

Bracklesham and Bournemouth Beds.

Lower Bagshot Sands and London Clay.

Lower Reading Beds and Thanet Sands.

1 "Geol. Mag.,” 1885, p. 293, and “ Historical Geology," 1886, p. 36. 2 "Manual of Geology," 1886, second edition, pp. 1120, 1164.

3 This chapter was written before Professor Prestwich read his paper "On the Correlation of the Eocene Strata" (" Geol. Soc. Proc.," Dec. 21, 1887), in which he proposes the same division of the Eocene series.

The Oligocene series may also be simply divided into :—

Upper Hempstead Beds,
Bembridge Beds.

Lower-Headon Beds.

The Hantonian strata of north-western Europe occur in several broad basins or trough-shaped areas separated by parallel anticlines or regions of elevation. The most

northern of these is that which we know as the "London basin," but which is really only part of a large troughshaped area extending from Belgium across the North Sea, and terminating in Wiltshire; the second is known as the Hampshire basin, and a third as the Paris basin.

A study of all these areas is necessary for a proper comprehension of the geographical changes which took place during the period, and the stratigraphy of the Paris basin is especially important from the fact of that area lying nearer to the southern shore of the Hantonian sea.

§ 1. Stratigraphical Evidence.

Eocene. The gap between the Chalk and the Tertiaries is not bridged over by beds of passage in any part of England, but in Belgium and Denmark there are deposits which show that the Cretaceous period was brought to a close by a general upheaval of western Europe-a change which led to the expulsion of the Cretaceous fauna from the European area, and the introduction of a very different shallow-water assemblage, which we call the Eocene fauna because it contains the ancestors of our modern shallowwater European species.

In England, therefore, there is a considerable break between the Cretaceous and Hantonian systems, but the upheaval which caused this break seems to have been such a gradual and uniform movement that it did not lead to

any widespread unconformity, producing rather what may be called a physical and paleontological hiatus, which is intensified by the contrast between the characters of the Chalk and the London Tertiaries. There is, however, a certain amount of unconformity, arising probably from unequal uplift and erosion during the unrepresented time, for, as Dr. Ch. Barrois has pointed out,1 "the centre of the Tertiary basin of London does not correspond with the centre of the Cretaceous basin of London. Norfolk, the deepest part of the Cretaceous sea, would still have been a depressed region after the Cretaceous if an upheaval of this area or a greater depression of the southern part of the basin had not modified this state of things before the deposition of the Thanet Beds."

A consideration of the variations in the thickness of the Chalk, as proved by deep borings in the London basin, shows that the Chalk is thinnest beneath or near London, and that it thickens in every direction from that centre, as if the original uplift had produced a low and broad dome which had its centre on the Surrey side of the Thames, and was quite independent of the subsequent upheaval which produced the axis of the Weald. Thus the Chalk is thinnest at Streatham (623 feet); beneath London, and thence northward as far as Loughton, it varies between 645 and 656 feet; further north it thickens to 680 at Cheshunt, and north-eastward to over 700 feet at Wickham Bishop and to 890 at Harwich; from London eastward it thickens to 682 at Chatham, and is over 734 at Chartham, near Canterbury; westward it is rather thicker at Richmond (671 feet) than below London, and at Bushey, to the northwest, it is about 700 feet. South-west, at East Horsley, between Leatherhead and Guildford, it is 817 feet, and on the

2

"Recherches sur le Terr. Cret. Sup.," 1876,

p. 179.

2 This thickness is obtained by adding the difference between the level of the ground at the well and the height of the Tertiary outcrop

south of the Weald, near Brighton, it is as much as 958 feet.

I agree with Dr. Barrois in thinking that the highest zones of the Chalk do not exist beneath the London area, but entirely differ from his view that their absence is due to original non-deposition. I see no reason to doubt that such an oceanic deposit as the uppermost Chalk extended continuously from Kent to Norfolk, and that its present absence near London is the result of pre-Tertiary erosion.

The lowest Eocene group, the Lower London Tertiaries, as they are sometimes called, is a somewhat complex one. The lowest member (Thanet Beds) is in East Kent about 80 feet thick, and consists chiefly of sandy marl or clay passing up into grey sand, with layers of calcareous sandstone; westward these beds are replaced by sharp grey or buff sand, which is 40 feet thick near Woolwich, but thins westward through Surrey, and has not been traced continuously further than Leatherhead. Beneath the London Clay they extend as far as Chertsey, and thence the boundary appears to run north-east along a line by Hampstead, Enfield, Epping, and Braintree, to beyond Sudbury. The Thanet Sands are therefore limited to the eastern part of the London basin.

The next subdivision is known as the Woolwich and Reading Beds, and is a very variable set of deposits, but has a much wider extension, for it occurs throughout the London and Hampshire basins, and seems to have been continuous originally across the intervening chalk ridges of Hampshire. These beds exhibit three distinct types or facies, and the geographical position and extent of these types is of course an important consideration.

1. The first and most widely distributed type is that of the Reading Beds, which prevail throughout the central to the thickness of chalk pierced by the boring. For this and other information on these borings I am indebted to Mr. Whitaker.