lying nearly horizontal, and stretching out at the base into an extensive flat pavement, on which the sea washes, and which is laid bare to a considerable distance at low water. Mingled with this clay is a large quantity of iron pyrites, which is sometimes so abundant that spontaneous combustion takes place when the beds are exposed to atmospheric action. The clay itself is of a silky lustre, and splits very readily into thin laminæ, but is sometimes extremely hard when dry. The shale is roasted until the sulphur of the pyrites combines with the alumina and potash, and forms a soluble salt (Alum), which is manufactured at Whitby and Redcar.1

The Upper Lias clay is frequently used for making bricks, tiles and drain pipes.

1 These remarks are taken from Prof. Ansted's Geology.

CHAPTER VII.

OOLITIC.

THIS term indicates the characteristic features of many of the limestones in the group, which have an oolitic structure. Oolite or roe-stone (derived from the Greek oon, an egg, and lithos, stone) is a name applied to limestones composed of small round particles of calcareous matter, which are cemented together and resemble the roe of a fish. When these particles approach the size of a pea the rock is termed Pisolite, Pisolitic limestone, or Pea-stone.

Thus we have the Inferior Oolite, the Great or Bath Oolite, the Oxford Oolite, and the Portland Oolite; also the Peagrit, a local member of the Inferior Oolite.

According to Prof. Phillips, the formation of oolitic grains seems to have followed on the accumulation of calcareous mud; in some cases the whole of this mud has become oolitic, in others segregations of distant sphericles took place. In general the small masses appear to have been gathered by attraction out of calcareous mud around nuclei of previously solidified matter-minute fragments of coral, echinida, foraminifera, and various shells-and, in some cases, grains of sand. But in other cases the oolitic structure seems to be independent of any internal nucleus, and must have been produced by chemical and molecular changes in the rocks.

The oolites, although characteristic of the series, by no means constitute its bulk: they are interstratified with sands and clays, and seemingly in places in so regular a sequence of sand, clay, and limestone, that much remark has been

made upon the subject. But this ternary or tripartite series is after all very local; the divisions that are made in the south of England do not correspond lithologically with those made in the north of Oxfordshire and the southern part of Northamptonshire, nor with those made in the northern part of Northamptonshire and Lincolnshire; whilst a comparison between the Yorkshire Oolites' and those of the Cotteswold Hills shows more strikingly the changes undergone by the strata, for in Yorkshire the beds are essentially arenaceous.

6

Prof. Hull has put forward reasons for concluding that all the Lower Jurassic formations exposed in the western and central counties die out towards the south-east; that this attenuation is due to the increase of distance from the sources of supply, and the consequent failure of sedimentary materials which were derived from land occupying the region of the North Atlantic. Such attenuation may in some cases, as pointed out by Mr. Topley and Mr. Lucy, modify our estimates of the dip.

As before observed, the persistence of the different types of strata varies much; the development of the oolites and sandy strata being more restricted than that of the clays. And it happens also in places that where one calcareous division is well developed, another in the same district becomes attenuated, as is the case with some of the Silurian limestones. Thus near Bath, where the Great Oolite is so well displayed, the Inferior Oolite is not so conspicuous; while the latter gains importance in its southward extension into Dorsetshire, where the Great Oolite has died out. Again the Lincolnshire limestone (Inferior Oolite) is more largely and 'prominently developed where the Great Oolite has a comparatively feeble representation. The Kelloway Rock and the Cornbrash may be cited as being amongst the most persistent of the calcareous strata.

The Oolitic strata appear to rest quite conformably upon

the Liassic sediments, but the whole Jurassic series exhibits many and great variations, and the lower beds of the Oolitic series sometimes appear to rest directly upon strata containing remains of Lower Lias, as types in Somersetshire.

It is indeed questionable whether there is any evidence of great unconformability in the Oolitic series in England, and whether the absence of individual members in certain places may not be due to local changes of condition, and in some cases perhaps to paucity of sediment: to conditions, in fact, which would affect the character both of the sediment and of the organic remains. Pauses in deposition are indicated at all horizons in some of the limestones; but there are no positive evidences over any extensive area, of the removal of any absent division, by denudation in Oolitic times.

Most of the strata are truly marine, but there are in places evidences of of estuarine conditions; and Prof. Phillips considers that in Yorkshire the strata give evidence of a true coal-field in the Oolitic era, produced by the interposition of vast quantities of sedimentary deposits brought down by floods from the land, between the more exclusively marine strata of the ordinary Oolitic type.

LOWER OOLITIC.

The following divisions have been established by the Geological Survey so far as the Midland and South-Western Counties of England are concerned: those in Yorkshire are given on the authority of Prof. Phillips, Dr. Wright, and Mr. Hudleston.' The different beds cannot be correlated with precision; the numbers refer to those which may roughly be considered as homotaxeous.

1 It is only right to state that this Classification was primarily based on the labours of William Smith, followed by Lonsdale.

Table showing the Subdivisions of the Lower Oolitic Rocks.

Upper Lias Clay.

Upper Lias Clay.

Mr. S. Sharp.