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Sulphuretted hydrogen, produced by the decomposition of iron-pyrites and water, is sometimes rather troublesome to well-sinkers, and Carbonic acid gas (Choke damp) is apt to accumulate in tunnels and borings in Chalk.
The Chiltern Hills and the North and South Downs are everywhere celebrated for the extent of their prospects. The boldness of the escarpment and the whiteness of the substance have given the idea of ornamenting the country in various parts by cutting away the turf. The White Horse, above Uffington in Berkshire, occupies about an acre of ground, and may be seen from some points of view at the distance of twelve miles. There is another land-mark of the same kind at Chervil, near Calne in Wiltshire, and a third in the neighbourhood of Thetford. On the chalk hill that faces Weymouth is a representation of George III. on horseback. Near Cerne is a figure of a giant holding a club in one hand and extending the other; this colossal figure is 180 feet in height. Near Prince's Risborough a large Cross has been cut.1
a series of tanks or reservoirs connected with each other. The sediment is formed into cakes and dried. (H. W. Bristow.)
1 These notes are partly taken from Conybeare and Phillips, Geology of England and Wales.
TERTIARY OR KAINOZOIC.1
EOCENE, MIOCENE, AND PLIOCENE.
OUR English Tertiary strata when looked at in a large way stand out in marked contrast to the Secondary and Palæozoic strata by reason of their lithological characters. They are composed for the most part of soft clays, and sands, with occasional pebble-beds. The marked lines of stratification so conspicuous in most of the Secondary strata are not recognized, and naturally, because the beds are less consolidated. We see in them the transitional strata between the rocky or stony formations and the recent deposits. The organic remains now begin to approach very closely to existing types: every stage brings us into contact with forms nearer to those now living.
It was for this reason that Lyell proposed the terms Eocene, Miocene, and Pliocene for the three great divisions into which European Tertiary deposits had been divided. These terms were based upon the percentage of recent mollusca found in the strata to which they were applied. Thus the Eocene strata (dawn of recent) contain a very small proportion of living species; the Miocene strata (less recent), although containing more recent species, yet contain a minor
The term Kainozoic (Cainozoic, or Canozoic) signifies recent life.
proportion compared to the Pliocene strata (more recent), which contain a plurality of recent species.
The Tertiary strata are characterized by numerous genera of large Mammalia, many genera and species of Mollusca identical with those now living, and dicotyledonous plants.
The Eocene strata have been divided and sub-divided as follows, and mainly from the researches of Prestwich, and Edward Forbes :
Upper Bagshot Sand.
The Hempstead Beds have by some geologists been regarded as
The Eocene strata occupy two distinct tracts, termed respectively the London and Hampshire Basins. The basinlike arrangement of the Chalk strata upon which these newer deposits lie, is, perhaps, most conspicuous in the Hampshire district, which includes the Tertiary beds of Dorset, Hants, the Isle of Wight, and Sussex. In the London Basin the sea-ward extension of the strata on the east is unknown, but wherever observed the beds rest upon a foundation or basin of Chalk. The formation of these Basins is due to upheaval and denudation subsequent to the Eocene period.
Although there is nowhere any marked unconformity in stratification between the Chalk and overlying Tertiary strata due to erosion in pre-Tertiary times, yet the change from deep-sea conditions to those of comparatively shallow-water is abrupt the pipes,' or irregular-shaped hollows, that have been dissolved out of the Chalk by the action of carbonated water, are features that may have been produced after the deposition of the newer strata. Nevertheless most of the lower Tertiary beds contain pebbles formed of flint evidently due to destruction of the Chalk; and when we also take into consideration the fact that in Denmark and Holland there are passage-beds between the Cretaceous and Tertiary formations which we do not find to be represented in England, the natural conclusion is that in our country these two great groups of rocks are unconformable.