At Bridport, on the coast of Dorsetshire, the Fuller's-Earth attains a thickness of 150 feet; and near Bath, where it forms a conspicuous band, it is 140 feet thick; at Wotton-under-Edge, 128 feet; near Stroud, 70 feet; and in the north of Gloucestershire, as near Cheltenham, it is further reduced to a general thickness of from thirty to forty feet. It appears at Sherborne, near Burford, to the northeast of Cheltenham, and does not extend as far east as Oxfordshire, beyond which the Stonesfield slate rests on the Inferior Oolite. The horizontal extension of the typical Fuller's-Earth in the southwest of England, which is about 180 miles, is very much the same as that of the Upper Lias Sands-preserving a maximum thickness from Bridport to Bath, but attenuating rapidly to the north and east, and finally thinning out a little to the north-east of Cheltenham.

Lithology.-The peculiar mineral from which the formation derived its appellation, is confined to particular districts, as around Bath and Stroud, and where it does occur, constitutes but a very small portion of the thickness of the formation. This Fuller's-Earth is used in fulling cloth. The lithological characters of the formation may be gathered from the following sections:

1. Cliff at Watton Hill, west of Bridport Harbour, "composed principally of blue clay, grey marl, and marlstone, with subordinate beds of imperfect stone; thickness of 150 feet; the base reposes on the Inferior Oolite, and the formation is covered by the ForestMarble."*

2. Bath, in descending order:—

A. Blue and yellow clay, with nodules of indurated marl 30
B. Bad Fuller's-Earth

C. Good Fuller's-Earth

Feet.

40

3

5

..

135.5-148

D. Clay, containing beds of bad Fuller's-Earth and layers
of nodular limestone and indurated marl

Bed D encloses one or two strata of a tough, rubbly limestone, which is commonly called Fuller's-Earth rock, and bears considerable resemblance to Cornbrash. This rock is always accompanied by an immense number of Terebratulæ, and Mya angulifera and Isocardia concentrica are almost invariably found in it.f 3. Slaughterford, East Gloucestershire:

White marls, with occasional stonybands

White and grey limestone, and Fuller's-Earth rock
White and blue calcareous clays with Terebratulæ

Feet.

4. Near Cheltenham.-The deposit consists of regularly bedded blue and yellow shales, clays and marls, with occasional courses of

* De la Beche and Buckland, Trans. Geol. Soc.,' 2nd series, vol. iii. (1830). † Lonsdale, Geol. of Bath, Trans. Geol. Soc.'

Hull, Mem. Geol. Surv., sheet 34.

rubbly limestone or calcareous sandstone. Most of the limestones are a lumachelle, some of them being entirely composed of Ostrea acuminata cemented together."

Paleontological Features of the Typical Fuller's-Earth.—The Fuller's-Earth does not possess a special fauna, and though regarded usually sterile as to the number of species, yet it is characterized by the profusion of Ostrea acuminata, Terebratula ornithocephala, and Rhynchonella varians. Professor Ramsay † summarizes the species of the Fuller's-Earth as follows:-Echinodermata 1, Conchifera 17, Brachiopoda 4. Total species 22, and remarks thereon:-" The majority of the forms that passed upwards from the Inferior Oolite limestone seem to have fled the muddy bottom of the Fuller's-Earth sea, and to have returned to the same area when the later period of the great oolite began The Fuller's-Earth may be considered only as a comparatively unfossiliferous and inconstant lower zone of the Great Oolite." The inference stated in the first sentence of the quotation is not consonant with facts; for in the first place that portion of the Inferior Oolite which has furnished the larger proportion of Great Oolite species, and indicates similar conditions of deposition, is that of lower freestone beds and pea grit of Cheltenham, which occupy the base of the Inferior Oolite, and in the second place, of the species common to the Inferior Oolite and Great Oolite, those which appear in the later stages of the former formation, occur also in the Fuller's-Earth. The second sentence of the above quotation is no longer applicable, inasmuch as though the fauna of the Fuller's-Earth is not exceedingly rich, yet far exceeds in number the then catalogued species, and, as I shall endeavour to show, has a greater affinity to the Inferior Oolite than to the Great Oolite.

The number of species catalogued by me from the Fuller's-Earth is 93, distributed in the following classes :

So that the Fuller's-Earth is not so barren in species as is generally supposed. Of the 93 recorded species, two, Montlivaltia tenuilamellosa and Belemnites parallelus, have not been found in any other formation; and respecting the range of Myacites Terquemus, Buv, and Pholadomya truncata, Buckman, I have no information; 6 forms are at present undetermined, whilst the bulk of the species, 81, occur in either the Inferior Oolite or Great Oolite, or in both formations.

*Hull, Mem. Geol. Surv., sheet 34, p. 11.

† Anniversary Address Geol. Soc., 1864.

See Lycett Cotteswold Club, vol. i., p. 71 et seq.; and Wright, Q. J. G. S., vol. xvi., p. 11,

The range of the 84 determined species is as follows:—

Accordingly, 69 species, or 83 per cent., occur in the Inferior Oolite, and 49 only, or 60 per cent., are found in the Great Oolite or superior formations. These results lead us to the inevitable conclusion, that the Fuller's-Earth is a subordinate member to the Inferior Oolite rather than to the Great Oolite, as hitherto considered.

The Inferior Oolite presents in its organic remains at least a twofold aspect, and whilst the older fauna is to some extent repeated in the Great Oolite, the newer fauna lived on during the deposition of the Fuller's-Earth, but did not extend into the Great Oolite. Indeed, the majority of the species common to the Fuller'sEarth and the Inferior Oolite made their appearance in time in the upper zones of the Inferior Oolite; such are:-Serpula tetragona, Holectypus depressus, Hyboclypus gibberulus, Clypeus Plotti, C. Hugii, Echinobrissus clunicularis, Waldheimia ornithocephala, Terebratula perovalis, T. globata, Rhynchonella varians, R. spinosa, Cypricardia Bathonica, Ceromya Bajociana, C. striata, Goniomya angulifera, Isocardia nitida, Mytilus Sowerbyanus, Ostrea acuminata, Pleuromya æquata, Pholadomya Heraulti, Ammonites Parkinsoni, A. discus, &c., &c.

And so close is the affinity subsisting between the fauna of the zone of Ammonites Parkinsoni and the Fuller's-Earth, that massing the species of these two horizons, the contrast in their numerical relations to those of the lower zones of the Inferior Oolite is as great as that which they present to those of the Great Oolite.

In conclusion, the Fuller's-Earth is not a formation, but only the uppermost, or a fourth, zone of the Inferior Oolite. Here, there is a marked decadence of Inferior Oolite species, yet at the same time the facies is decidedly after that fauna.

NOTICES OF SCIENTIFIC WORKS.

WROUGHT-IRON BRIDGES AND ROOFS.*

Or all materials used in construction in these days of progress, there is none which plays so important a part in engineering and architectural art as iron. This material has become the only resource of the engineer for carrying out those large designs and projects which thirty years ago would have been considered chimerical. The attempt to cross the Menai and Conway Straits with bridges formed of wrought-iron plates was treated by mathematicians and engineers as they would have regarded a chapter in the Arabian Nights; and more than one eminent mathematician pronounced the attempt however ingenious the combinations might be in the shape of iron plates-as a wild and fabulous scheme. It was only those connected with the preliminary experiments-which led to the form and principle of construction-who could form a correct idea of the project and establish with certainty the principle on which those important structures were founded.

It must be admitted that the position of the load and the form of its distribution were not attended to with the same mathematical accuracy as at the present day; but the general dimensions, extent of span, and weight of load were carefully considered, in order to resist every possible strain and to render the structures secure. All this was done in the face of detractors, and the doubts and fears of men of science, and the results are the completion of the Britannia and Conway Tubular Bridges, as they now exist, as firm and secure as the first day they were opened for public traffic. It is true that tubular bridges, such as the Britannia and Conway, are of a more expensive type than those subsequently introduced; but although extended practice and the progress of science may have suggested improvements, it cannot be disputed that all subsequent constructions of this kind are founded on the same principles as those to which we may safely refer as the pioneers of all their successors. We have a much greater variety in the forms of wrought-iron bridges now than when first introduced; but the principle, so clearly exemplified in the Britannia and Conway Tubular Bridges, is identical with more recent constructions in the balance of the two resisting forces of tension and compression, as exhibited in the upper and lower sides or flanges of those important structures.

*Wrought-iron Bridges and Roofs:' Lectures delivered at the Royal Engineer Establishment, Chatham. By W. Cawthorne Unwin, B.Sc. Associate Inst. C.E. E. & F. N. Spon.

Wrought-iron girders, whether tubular, plate, or lattice, are therefore simply modifications of what has already been done in the same direction, and provided careful attention is given to the quality of the material and soundness of the workmanship, we may rest assured of the security and permanence of every similar structure.

Many distinguished men of science have written treatises to elucidate the principles involved in the tubular and girder constructions, and among them may be mentioned Mr. Unwin, who has just published a report of his lectures on this subject, including a short treatise on the construction of iron roofs, delivered to the members of the Ordnance Corps at Chatham. As a lecturer on. practical science a more efficient person could probably not have been selected, for-to a competent knowledge of mathematics applied to constructions-Mr. Unwin has had the advantage of five years' practical experience with Sir William Fairbairn of Manchester, and he therefore enters upon the discussion with a full knowledge of the subject on which he treats.

In the first lecture Mr. Unwin fully proves the advantages he has gained, by the competency with which he enters upon the investigation of stress and strain, bringing to his aid many apt illustrations, and by the graphic way in which he treats the principles of load and molecular resistances, greatly for the benefit of the student and those interested in the accuracy of iron constructions. To show the distinction between stress and strain, which means the force of the former applied to a material body, and the alteration or the resistance of the latter as a result, Mr. Unwin states, that the "strain is sensibly proportional to the stress for a range of about only one-third of the whole stress which may be applied before rupture ensues, if the bar has not previously been strained, and for a range of, perhaps, two-thirds of the breaking-stress, if the bar has not previously been loaded with nearly the whole breaking-weight. But in either case, with loads near the breaking-weight, the strain is not proportional to the stress, and the condition of perfect elasticity is not fulfilled. Hence laws derived from the consideration of a perfectly elastic material will not give accurately the ultimate resistance of structures of wrought iron."

The second lecture is devoted to the intensity of stress on bridges, and the methods of estimating the load and its limits of safety. It also relates to the weight of the structure and its load, or the dead weight and its live or rolling load. In this, as in the former lecture, Mr. Unwin gives some useful examples and illustrations of highly practical value to the student and engineer.

The third lecture treats of tubular, tubular-girder, and plategirder bridges, showing the ratio of the top and bottom flanges to span, the depth of girder to span, and the method of designing, &c.