come when an effort should be made to give more completeness to our knowledge of the Calamita generally, and especially to clear up the points which are still doubtful in the organisation of Arthropitys.

9

THE MODE OF DEPOSITION AND PROPERTIES OF THE CARBONIFEROUS STRATA OF LEEDS AND ITS IMMEDIATE SUBURBS.

BY BENJ. HOLGATE, F.G.S.

It is well known that Leeds stands in an enviable position as regards its minerals, and owes its prosperity in a great measure to them. It is situated on the northern edge of the Yorkshire Coalfield, and consequently on the Lower Coal Measures, it is rich in coarse and fine-grained sandstones. Some of these are of the greatest durability when exposed to weather and other decaying influences, and are used for building and monumental purposes. Others are suitable for grindstones, the grains of silex not being so firmly bound together. These various kinds of stone are obtained on the north and east, within the Borough, and are well known to architects, engineers, and builders throughout the United Kingdom. The coals, of which there are no less than five good workable seams dealt with here, besides numerous others in the overlying strata, embrace almost every variety, and are used for every purpose to which coals are put. Firebricks and pottery made from the clays are exported to numerous foreign ports.

The carboniferous strata between the coarse millstone grit and the fine Elland flagstone, both alluded to above, have thinned out to the north of the town, and though the ganister measures and their included coals are all present, they are much thinner than in the districts to the south-west, in the neighbourhood of Halifax. The strata have an almost uniform dip towards the south-east, and have not been contorted notwithstanding their great age, although faults are numerous. Almost every cubic foot of the strata of the coal measures is made use of, the stones for building purposes, the coals for generating heat, the ironstone for making iron, the fireclays for making firebricks to withstand high temperatures, terra cotta and faience ware, the inferior fireclay for coarse pottery, and the remainder, consisting of black shale, blue bind, brown and grey binds, and soft stones, are used for making the ordinary common and pressed bricks, of which Leeds is principally built.

B

Open quarries are worked to the south and east of the town, some of which are 70 feet in depth. As they are always being worked they constantly present a fresh face for study. Commencing on the east side of the town some feet below the base of the workable coal seam known as the "Better Bed" (the one lying next above the Elland Flagstone), and going from quarry to quarry, we may follow in an upward direction an almost uninterrupted succession of measures as they are laid bare through a vertical depth of upwards of 300 feet. This is owing to the dip of the strata, the quarries being at an almost equal elevation above the sea. The conditions have been very varied under which these 300 feet of strata were laid down. There are few geological facts which are not here illustrated: land deposits and deposits made in almost still water or left by rapidly flowing streams, also plant and animal remains in immense quantities, though as far as can be made out of not many different kinds; an examination however leads us to suspect that the varieties have been much more numerous, the traces having been obliterated or altered by the numerous changes that have taken place during the immense period of time that has elapsed since their deposition, so that none but the most indestructible forms are preserved, unless they are embedded in modules. The land has been repeatedly elevated and depressed, the chemical action of water permeating through the mass, here taking up elements in solution and there depositing them under different conditions, or exchanging them for other elements has also had an important share in the change. To speak of all the phases of these changes would be a great though an interesting task. I must content myself with giving a table briefly describing the different kinds of strata, and will only enlarge upon a few typical ones.

WATERS.-There are many soluble salts present in the shales and binds producing mineral waters of different kinds. The sandstones allow the water to pass freely through them with but little. change, and serve only as a mode of conveyance from one stratum of shale to another, so that they do not retain the salts. We find different kinds of mineral waters at different horizons; some are medicinal, such as that known as the Holbeck Spa Water; others

contain a large quantity of salt in solution, as do those immediately above the Beeston Bed Coal (See Tables Nos. 1 and 45.)

SEAT EARTHS.-These underlie each seam of coal however thin it may be, and are generally about 2 feet to 3 feet in thickness, from which we gather that this was the distance to which the coal measure trees rooted. Like the trees of the present day those of the coal measures contrived to exist upon different kinds of soil whatever their preferences might be, and we find some rooting in mud as in that beneath the Beeston Bed (Table No. 40.) Others in loamy sand (see Table No. 18) as is the Black Bed Seat Earth, and others (No. 27) again in a micacious bind. In the Better Bed Seat Earth (No. 2) which is the fireclay for which Leeds is celebrated, the roots completely destroyed all stratification, and made a soil just as trees do at the present day. In some, however, this has not taken place, and though the roots and rootlets are numerous we can distinctly see what the original deposit was like (See Table No. 27). In this case we find a micacious and bituminous bind.

On examination we are struck with the fact that the thickness of the coal seams do not appear to bear any proportion to the depth of, or the quantity of roots in the seat earth; a seam of coal 2 in. in thickness has as great a depth of seat earth and often as many roots as a coal seam having a thickness of some feet. If, however, we examine the overlying bed we shall see the reason of this, and shall find it to be for some feet in height, a shale, very black and bituminous. We shall also probably find stems of trees standing up in it. If we burn this shale it will give out a considerable amount of heat, and will lose a great deal of its weight, sometimes as much as two-thirds. From this we realise that the land was sinking at a greater rate than the trees and plants were growing, and that a coal seam does not represent the total amount of growth, but only that part of it which is made up entirely, or very nearly so, of vegetable matter. The trees and plants stood and grew for a long time after this, during which time mud was being brought down by streams and deposited in the spaces intervening between the plants. The time when the forest was quite destroyed is marked by a difference in the colour and in the nature of the deposits. The strata containing

floated fronds of ferns and calamites are blue in colour, and not so

dense or tough.

We also realise that even if our coals were to run short, we should still have beds of immense thickness which, though not so valuable as pure coals, might in case of need be made to render good service (See Table No. 20, 50, etc.)

The sharp line of demarcation between the coal and the overlying stratum has most probably been produced by such stratum sliding over the coal during the elevations and depressions that have taken place since they were deposited, and by the different distances apart at which the force producing lateral pressure has caused the strata to joint or cleave.

Sometimes we find that the trees actually took root under water and grew for some time whilst a considerable amount of mud was being deposited; that the mud afterwards ceased to be brought down and that consequently the coals are pure. We thus have an impure coal immediately above the seat earth and below the pure coal. This is what has taken place in the Beeston Bed (Table No. 41). For about two feet between the seat earth and the coal at present worked is a stratum which is left unworked at the present time. Years ago this was sold at a low price to the very poor under the name of "Doggies." It is a very heavy and dirty coal. At Churwell it is known as the "Churwell thick," and possesses the same properties. Here the Beeston Bed actually divides into two distinct coal seams, the distance between the upper and the lower part increasing from nothing up to a distance of ten yards, with intervening measures of shale, showing that at Churwell the forest was completely overwhelmed by the mud which had been brought down by the water, but that at Hunslet it was left free from mud to grow with greater purity. This it did, making a coal seam 8ft. in thickness but with several mud partings. It is interesting to know that the latter continued growing, whilst that at Churwell was being covered up with mud to the height above named and that the forest again extended itself over the Churwell area, the upper part of the Beeston Bed making the Upper Churwell thin coal. There are few coal seams that did not vary in character during