Four years have elapsed since I first commenced my researches among the Carboniferous Brachiopoda of Great Britain ; and I should certainly by this time have completed my task, had not the unfortunate delay in the publication of the last two or three volumes of the Palæontographical Society induced me to undertake other work which would not require to lie printed and unpublished for upwards of one year and a half. My monograph cannot, consequently, be completed or entirely published for some time to come, perhaps a year or more; but as my researches in connection with the subject are almost ended, since the whole series of species at present known have been as carefully examined as my means and materials would permit, it may, perhaps, be as well that I should at once expose the results of my laborious enquiry, in the hope that by so doing some further assistance and advice may be proffered; which might enable me to make the monograph still more complete, and at the same time admit of my correcting in the concluding pages those unavoidable mistakes which have been commited during the interval which has elapsed since the commencement of its publication.

It may be thought by some while perusing the accompanying catalogue that the work to be gone through was but small in comparison with the time employed, but such would be an erroneous assumption, and a sad return to the numerous friends in England, Scotland, and Ireland, who have so zealously afforded their valuable and valued assistance, by incessantly ransacking the country in order to obtain every possible specimen that might assist and tend to complete the history of British Carboniferous species.

In our last lecture stress was laid on the fact that coal-beds, unlike mineral veins, are stratified—not injected, or filling cracks in the earth as metals do. And when we use the term stratified, we mean that the materials we are considering—coal, ironstone, sandstone, clay, shale—were all deposited sheet over sheet, layer over layer, principally by the agency of water.

In scarcely any other way, except by water, can we conceive of materials being spread abroad over vast surfaces, in that even and regular manner which we call “stratified.” As a rule, the matters ejected from the mouths of fiery volcanos are only rudely heaped up, and unless they fall into the sea, do not undergo this smoothing, spreading-out process. The sand of the soa-shore however, and the pebbles on its margin, and the mud of its great depths, are truly “stratified;” and if a fertile plain, or a marshy district were submerged in the waters, the materials on that surface would be soon covered over by the ooze and sand and shingle, and would then be said to be “interstratified” with them. In this way coal-beds occur among beds of sandstone and other rocks.

It is seldom that any coal-field contains more than twenty-five or thirty workable seams: and perhaps these altogether do not amount to above eighty or one hundred feet at the utmost, while in South Wales the coal strata are twelve thousand feet thick. The mass, you see, is rock.