These beds, which vary much in thickness, lie in North Durham in the general form of a syncline beneath Sunderland. The unfossiliferous Yellow Sands are probably a deltaic formation reassorted by wind, the other beds being the result of deposition in an inland sea undergoing desiccation. The magnesium carbonate existed in the waters of the sea, and was either deposited along with the calcium carbonate, or introduced by seepage when the beds were being laid down. Great changes in the amount and distribution of these carbonates have, however, taken place since deposition. The cellular structures that occur in the limestone can be classified as follows:—(1) Concretionary-cellular; (2) negative breccia; (3) solution-cavities; and (4) fractured cellular. Most of them have been produced by the leaching-out of the magnesium carbonate (dedolomitization), or of both that and calcium carbonate. In some cases the rock has been rendered crystalline, as well as more calcareous, and the fossils have been obliterated. They do not afford any proof that the rock has been dolomitized subsequent to deposition. The percentage of calcium carbonate is sometimes over 99, while that of magnesium carbonate is occasionally as much as 50. The fauna of the Magnesian Limestone is very restricted (about 140 species) and most peculiarly distributed. The marked paleontological features are the profusion of individuals in the Middle Fossiliferous Limestone (which appears to have formed a shell-bank in the Middle Magnesian-Limestone sea) and their sudden disappearance in the Upper Limestone. No corals, echinoderms, polyzoa, brachiopods., or cephalopods have ever