good a fit in size as in form; and the bulk of the nodule bears almost always a definite proportion to the amount of animal matter round which it had formed. I was a good deal struck, a few weeks ago, in glancing over a series of experiments conducted for a different purpose by a lady of singular ingenuity,--Mrs. Marshall, the inventor and patentee of the beautiful marble-looking plaster, _Intonacco_,--to find what seemed a similar principle illustrated in the compositions of her various cements. These are all formed of a basis of lime, mixed in certain proportions with organic matter. The reader must be familiar with cements of this kind long known among the people, and much used in the repairing of broken pottery, such as a cement compounded of quicklime made of oyster shells, mixed up with a glue made of skim-milk cheese, and another cement made also of quicklime mixed up with the whites of eggs. In Mrs. Marshall's cements, the organic matter is variously compounded of both animal and vegetable substances, while the earth generally employed is sulphate of lime; and the result is a close-grained marble-like composition, considerably harder than the sulphate in its original crystalline state. She had deposited, in one set of her experiments, the calcareous earth, mixed up with sand, clay, and other extraneous matters, on some of the commoner molluscs of our shores; and universally found that the mass, incoherent everywhere else, had acquired solidity wherever it had been permeated by the animal matter of the molluscs. Each animal, in proportion to its size, is found to retain, as in the fossiliferous spindles of the Old Red Sandstone, its coherent nodule around it. One point in the natural phenomenon, however, still remains unillustrated by the experiments of Mrs. Marshall. We see in them the animal matter giving solidity to the lime in immediate contact with it; but we do not see it possessing any such affinity for it as to form, in an argillaceous compound, like that of the ichthyolite beds, a centre of attraction powerful enough to draw together the lime diffused throughout the mass. It still remains for the geologic chemist to discover on what principle masses of animal matter should form the attracting nuclei of limestone nodules.