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Rh we find in Buckinghamshire a marine and estuarine group divided as follows :

In parts of Northamptonshire and in Lincolnshire this group is overlain by an important division known as the Lincolnshire Limestone, which furnishes many a valuable freestone. No portion of it is repre- sented in Buckinghamshire, and consequently there is a considerable break between the representatives of the Inferior Oolite and Great Oolite Series in the county. Locally also there are evidences of erosion between the Upper Lias Clay and the succeeding Northampton Sands.

Just beyond the borders of the county, in a brickyard north-east ot Brackley, resting on the blue pyritic clays of the Upper Lias, there were to be seen green and dark grey sands and hard ferruginous sandstone, together 3 feet 6 inches thick. The stone contained Avicula braamburiensis and some other fossils, and also pebbles of hardened Upper Lias shale. These sandy beds, representing the Northampton Sands, were overlaid by 8 feet of purplish loam, clay and white and brown sand, perhaps belonging to the Lower Estuarine Series.

In this neighbourhood however the lower beds of the Great Oolite comprise an Upper Estuarine Series, and where the two Estuarine series come together it is most difficult to distinguish between them, for in characters they are alike, and it is only where one group is seen to rest with marked unconformity on the other that any division can be made. Further north in Northamptonshire the two are separated by the Lin- colnshire Limestone, but the Upper Estuarine Series may be regarded as the more persistent as it stretches unconformably across the eroded faces of the subdivisions in the Inferior Oolite Series.

The Northampton Sands are exposed in Buckinghamshire only along the Ouse valley at Whitfield Mill below Biddlesden. Hard bands such as occur near Brackley have been met with in borings on the north-east of Stowe Park near Akeley and along the borders of the Ouse valley near Stony Stratford ; but the evidence of their age is indecisive.

There is no doubt that both Northampton Sands and Lower Estuarine Series die out in a south-easterly direction from Northampton- shire towards Olney and Stony Stratford. Together they appear to represent in places the higher portion of the zone of Ammonites jurensis, but they consist mainly of the zones of A. opalinus and A. Muchisonae; or in other words they are equivalent to the higher part of the Midford Sands and the lower part of the Inferior Oolite of the west and south- west of England.

GREAT OOLITE SERIES This series, which occupies a considerable area in the northern part of the county, is locally divided as follows :