Various attempts have been made to measure the positive length of geological periods. Some geologists have sought to determine, roughly, the age of the stratified rocks by calculations based upon their probable thickness and the rate at which they may have been deposited. This method, however, is worthless, because the rates which have been adopted are purely arbitrary. One geologist will take the rate of deposit at a foot in a hundred years, while another will assume it to be a foot in a thousand or perhaps ten thousand years; and, for any reasons that have been assigned, the one rate is just as likely to be correct as the other: for if we examine what is taking place in the ocean bed at the present day, we shall find in some places a foot of sediment laid down in a year, while in other places a foot may not be deposited in a thousand years. The stratified rocks were evidently formed at all possible rates. When we speak of the rate of their formation, we must of course refer to the mean rate; and it is perfectly true that if we knew the thickness of these rocks and the mean rate at which they were deposited, we should have a ready means of determining their positive age. But there appears to be nearly as great uncertainty regarding the thickness of the sedimentary rocks as regarding the rate at which they were formed. No doubt we can roughly estimate their probable maximum thickness; for instance, Professor Ramsay has found, from actual measurement, that the sedimentary formations of Great Britain have a maximum thickness of upwards of 72,000 feet; but all such measurements give us no idea of their mean thickness. What is the mean thickness of the sedimentary rocks of the globe? On this point geology does not afford a definite answer. Whatever the present mean thickness of the sedimentary rocks of our globe may be, it must be small in comparison to the mean thickness of all the sedimentary rocks which have been formed. This is obvious from the fact that the sedimentary rocks of one age are partly formed from the destruction of the sedimentary rocks of former ages. From the Laurentian age down to the present day, the stratified rocks have been undergoing constant denudation.

I had the honour in 1866 to describe the first specimen of a fossil Myriapod from the Coal-measures met with in this country. It was discovered by the late Mr. Thomas Brown, of Glasgow, an indefatigable geologist and an ardent collector of fossils, who obtained it in a nodule of Clay-ironstone from Kilmaurs.

During last summer I visited the interesting and ancient town of Ludlow, where, under the guidance of Mr. Robert Lightbody, F.G.S., I had the opportunity of seeing the admirable Public Museum, belonging to the Ludlow Natural History Society, and also the rich private collection of Mr. Lightbody and that of Mr. Humphrey Salwey. In the Society's Museum, my attention was directed by Mr. Lightbody to a truly remarkable fossil from the collection of Mr. Henry Pardoe, and obtained by him from the Lower Ludlow, Church Hill, Leintwardine. It consists of seven body-segments and three caudal spines, of the largest example of Ceratiocaris I have ever seen; exceeding in size the C. Bohernicus of Barrande, or C. Murchisoni from the Upper Ludlow Rock, Ludlow and Wenlock Shale, Dudley, figured and described by me in the Geological Magazine, 1866, Vol. III., PI. X., Fig. 8, pp. 203–205. The figure (taken from a sketch made at the time) given on Plate III., Fig. 3, being only one-third the size of the fossil, conveys but a very inadequate idea of this interesting Crustacean remain. The seven body segments together measure eight and a half inches in length, and nearly two inches in breadth; they are nearly quadrangular, and are covered with fine and delicate parallel slightly waved striæ. The first segment is three-quarters of an inch long; the second, half an inch; the third, three-quarters of an inch; the fourth, fifth, and sixth, one inch each in length; and the seventh, two and three-quarter inches long.

It has always been difficult to account for the feet that, while Moraine Drifts occur at such low levels in the Lake District, Marine Drifts are found at such high elevations in the surrounding district. Mr. Mackintosh's new facts throw some light on this phenomena, but do not entirely explain it, owing to his having given, in my opinion, far too little importance to the second Glaciation experienced by the Lake District after its emergence from the Glacial Sea.

During the years 1866 and 1867 considerable discussion took place through the medium of this Journal on the question ofthe existence of dislocations, or faults, in Glacial Drift. As I believe no communication has appeared since the latter year in these columns on that subject, it may not be altogether uninteresting to your readers if I briefly refer to a clear and unmistakeable example of faulted drift which recently came under my observation at a ballast pit close to the Stockport Railway Station. At the time of my first visit, about three months ago, my object being to search for traces of marine shells in the sand, the several details of the section were exhibited with remarkable distinctness, the action of the weather having brought out the several lines of bedding and fracture as sharply as though they had been produced in a body of compact Sandstone; indeed, so exact was the resemblance in that respect that at a short distance it would have been difficult to believe that such was not the fact. Unfortunately, since that time, a quantity of sand has been removed from the face of the pit, thereby partially obliterating the details; the principal features, nevertheless, were easily discernible a few weeks ago, when I last visited the spot.

On the 30th July last year I observed that a deposit of Websterite, subsulphate of alumina, had been cut into, in excavating for the new system of drainage in the Montpelier Road opposite the south end of Vernon Terrace. It occurs at a depthof 16 feet from the surface of the road, beneath a ferruginous deposit of varying depth, which overlies the chalk on the summit of the hill, consisting of ochreous clay with occasional flint-breccia and masses of hæmatite iron ore in some instances mammillated and associated with crystals of selenite. The iron ore is occasionally friable and of a cindery appearance, containing in its cavities angular pieces of chalk and occasional groups of crystals of selenite. The deposit of Websterite is about three feet wide at its junction with the overlying ferruginous mass, narrowing as it descends, apparently occupying a fissure in the chalk, which has at some time been filled with clay, or has been formed by some decomposing action on the chalk, the chalk intruding occasionally into the vein of Websterite. The mineral varies much in colour and appearance, consisting in some places of a soft white powder, which, I am informed by Sir W. C. Trevelyan, hehas observed in specimens at Newhaven, and which he has ascertained by the microscope to consist entirely of minute transparent crystals, the nature of which he believes has not yet been investigated; sometimes in masses of various size presenting the appearance of meerschaum, compact and structureless, or somewhat botryoidal in form, occasionally presenting a concentric structure, and rarely and only in a certain portion of the deposit exhibiting spherical concretions with a radiating structure. Specimens of these various forms I have presented to the British Museum. A mass of yellow clay with imbeddedchalk flints divides the summit of the vein of Websterite, and near the clay the mineral assumes the character of allophane, having a yellow ivory-like appearance, towards the chalk forming the wall of the vein of Websterite. The wall of the vein is marked by a dark line caused by the association of a soft black substance, oxide of manganese, with the Websterite.