TABLE illustrating the VARIATIONS which the BEDS of the LOWER OOLITE undergo in the MIDLAND DISTRICT.

and it is in places almost wholly made up of beds of small oysters, a feature never presented by it in its normal aspect. Before we reach the Humber, the Cornbrash is found to have altogether thinned out and disappeared. The so-called Cornbrash of the North of Yorkshire is not only not continuous with that of the South and Midland districts of England, but, as shown by Dr. Lycett,* presents essential points of difference from that formation in its mineral character and still more striking ones in its fauna. It would be well if a local and distinctive name were applied to the Yorkshire rock, which is perhaps the only representative of the Great Oolite series in the northern area.

The Forest Marble, which was evidently a shallow-water deposit, and as Professor Phillips has shown, sometimes even exhibits estuarine characters, everywhere presents great variability in the succession and thickness of its various beds of clay, sand, and shelly limestone. In Oxfordshire the limestones thin out and disappear altogether, and the clays with occasional shelly bands, become so thin and insignificant in North Oxfordshire, South Northamptonshire, and the adjoining counties, that it was found impracticable by the Geological Survey to map them separately, and hence they are in those districts grouped with the Great Oolite. As we go northwards into North Northamptonshire and Lincolnshire these beds of clay again thicken, and become of greater importance, but they do not include the characteristic shelly limestones of the Forest Marble of the south of England. They are mapped in sheet 64, and that to the north of it, under the name of "Great Oolite Clays." It is true that the strata of the Upper Zone of the Great Oolite in the Midland district occasionally contain fissile limestones identical in character with those of the Forest Marble, but this is evidently the result of a local similarity of conditions, and neither paleontological nor stratigraphical evidence can be adduced in favour of considering them as part of that formation. The Bradford Clay is a more local and inconstant stratum even than the Forest Marble; important and interesting as are its characters in the Bath district, it loses almost all its importance in the Cotteswolds. The identity of the stratum at Tetbury with that at Bradford has even been doubted by some geologists, and it has been found quite impracticable by the Geological surveyors to map it as a separate formation.

The Upper Zone of the Great Oolite is, in its persistency and uniformity of character, only second to the Cornbrash itself.

Supplementary Monograph on the Mollusca from the Stonesfield Slate, Great Oolite, Forest Marble, and Cornbrash, by John Lycett, M.D. Published by Palæontographical Society, 1863, p. 117.

Everywhere exhibiting alternations of white marly limestones and clays, crowded with a highly distinctive fauna, in which the Myada, Ostreidæ, and Echinodermata are especially noticeable by their abundance both of species and individuals, these strata (which are constantly burnt for lime in the districts where they are developed) are well known to all who have studied the geology of the Northern Cotteswolds, Oxfordshire, Northamptonshire, and Lincolnshire. At times, it is true, they exhibit local variations; passing in some places into shelly and occasionally oolitic freestones which afford good building stones, and in others into fissile beds which present appearances similar to those of the Forest Marble. When the formation is traced through the country, however, the observer cannot doubt of the continuity of the series of beds. In Mid-Lincolnshire the Upper Zone of the Great Oolite is very greatly reduced in thickness, and its lower calcareous portion finally thins out and disappears altogether, at a point considerably to the south of that at which the Cornbrash is lost.

The Lower Zone of the Great Oolite, which, in its frequent oblique lamination, its numerous remains of terrestrial organisms, and the often prevailing arenaceous elements of its composition, suggests, like the Forest Marble above, with which indeed its beds were at one time confounded, the littoral conditions under which it was deposited. The thick shelly freestones of Minchinhampton Common pass northwards into fissile shelly and often sandy limestones, which in the Northern Cotteswolds and South Oxfordshire present at their base, in local patches, fissile beds; these at Eyeford, Sevenhampton Common, and Stonesfield are capable of being split, by the aid of frost, into "slates" used for roofing purposes. It was found by the Geological surveyors that these "slate yielding beds, as they are traced northwards, lose their calcareous characters and are represented by sands, which occasionally become ferruginous. As we shall hereafter show, certain beds of the Inferior Oolite undergo a precisely similar change of character in the same area; and the two series of sandy beds representing the attenuated and more littoral conditions of two important limestone formations of the Cotteswolds (namely the Leckhampton and Minchinhampton freestones) thus brought together, being frequently altogether destitute of fossils, the line of demarcation between them can no longer be traced. These sandy strata, which in places are reduced to only a few inches in thickness, have been mapped together under the name of the "Northampton Sand." As we go northwards, however, the sandy representative of the Lower Zone of the Great Oolite is found gradually to change in character and to become mainly argillaceous in its composition. These beds of clay are evidently of estuarine character

presenting alternations of bands with freshwater and marine fossils, and mineral characters identical with those of the Purbeck and the beds which form the top of the Wealden. These strata, which in North Northamptonshire and Lincolnshire present very marked characters, have been mapped by the Survey under the name of the "Upper Estuarine Series;" they form the base of the Great Oolite. The strata in question were first described by Professor Morris in 1853, from their exposures in the cuttings of the Great Northern Railway, then in course of construction; the beds were at that time however regarded as the equivalents of the Forest Marble. As we pass northwards in the county of Lincoln the Upper Estuarine Series, like the other members of the Great Oolite, becomes gradually reduced in thickness, and by the thinning out of the Upper Zone of the Great Oolite, the two argillaceous series, representing the Forest Marble and the Stonesfield Slate respectively, are brought together; thus the only vestige of the Great Oolite formation below the Cornbrash in North Lincolnshire is a thin series of clays of more or less estuarine character. It is doubtful whether any representative of these argillaceous beds extends to the north of the Humber.

The Fuller's Earth, which appears by its fauna to form a transition series between the Great Oolite and Inferior Oolite of the Cotteswolds, is a very variable member of the Jurassic series. Near Bath it is 150 feet thick, at Sapperton Tunnel only 70, and in the northern part of the Cotteswolds it thins out and disappears altogether.

The Ragstones of the Inferior Oolite, as shown by Dr. Lycett in his valuable" Handbook to the Cotteswold Hills," undergo many variations in character within that area. As we pass northward and westward into Oxfordshire, however, this portion of the Inferior Oolite no longer presents its well-marked subdivisions, but, as was shown by Mr. Hull, is represented only by the "Clypeus Grit," which, becoming gradually more and more reduced in thickness, finally disappears near Chipping Norton, and to the north of Witney. The Ragstones of the Inferior Oolite are shown by Dr. Wright to represent the Zone of Ammonites Parkinsoni.

The Upper Freestones, which in places are almost destitute of fossils, have been shown by Dr. Wright to be represented at Cleeve Hill by a series of strata, yielding the characteristic fossils of the Zone of Ammonites Humphresianus. This division is perhaps the least constant of all the beds of the Inferior Oolite of the Cotteswolds; besides undergoing numerous and rapid

Variation in Oolite strata. the Inferior

changes in mineral character, it thins out and disappears northwards and eastwards before even the Oolite Marl.

The Oolite Marl is a thin, but an interesting and well marked, stratum, which has been regarded by Dr. Wright as representing the upper part of the Zone of Ammonites Murchisonæ, but by Dr. Waagen has been referred to the Zone of Ammonites Sowerbyi. It can only be traced over the middle and northern parts of the Cotteswolds, and even within those areas undergoes very considerable changes in mineral character and thickness.

The Lower Freestones, which form so large a portion of the mass of the Inferior Oolites of the Cotteswolds, partake of the general attenuation of the beds of that series towards the north and east. Near their base the Lower Freestones become sandy and sometimes ferruginous, and thus graduate into the Pea Grit below. In their northern extension these sandy and ferruginous characters become still more marked, as may be well seen in the Great Outliers of Ebrington and Bredon Hills.

The Pea Grit, with the "roe-stone" in its upper part, and the sandy ferruginous beds at its base, has but a very limited range in the Northern Cotteswolds. North of Stanley Hill it can no longer be recognised as a distinct bed; but it may possibly be represented, together with the freestones above, in the sandy and ferruginous beds which constitute the base of the Inferior Oolite in those northern spurs and outliers of the Cotteswolds, known as Broadway, Campden, Ebrington, and Bredon Hills.

The Midford Sand appears to form a transition series between the Upper Lias Clays and the Inferior Oolite. At this horizon in Swabia there is developed a series of most richly fossiliferous beds distributed by the German geologists into two Zones--the Zone of Ammonites Jurensis and the Zone of Ammonites torulosus; and between these is the line which is generally accepted upon the continent as separating the Upper Lias from the Inferior Oolite. In this country, however, the strata at this horizon, though attaining a great thickness in some places, are almost wholly unfossiliferous, except in one or two thin bands; hence a division similar to that adopted by foreign geologists does not appear to be practicable in England. From a thickness of 150 ft. in the Southern Cotteswolds they thin out rapidly in going northward, and cannot be mapped as a separate bed farther in that direction than Chipping Campden; as pointed out however by Dr. Holl, there are some grounds for believing that, in the sandy and ferruginous beds of the extreme northern spurs of the Cotteswold range, the Midford Sand as well as the Pea Grit and the Lower Freestones are represented. The paleontological