In Dr. Carpenter's work on the microscope occurs the following:-- "Thus, when we meet with an extensive stratum of fossilised _Diatomaceæ_, in what is now dry land, we can entertain no doubt that this silicious deposit originally accumulated either at the bottom of a fresh-water lake or beneath the waters of the ocean, just as some deposits are formed at the present time, by the production and death of successive generations of these bodies, whose indestructible casings accumulate in the lapse of ages, so as to form layers, whose thickness is only limited by the time during which this process has been in action. "In like manner, when we meet with a limestone rock entirely composed of the calcareous shells of Foraminifera--some of them entire, others broken up into minute particles--we interpret the phenomenon by the fact that the dredgings obtained from certain parts of the ocean-bottom, consist almost entirely of remains of existing Foraminifera, in which entire shells--the animals of which may be yet alive--are mingled with the debris of others that have been reduced by the action of the waves to a fragmentary state. "Now, in the fine white mud which is brought up from almost every part of the sea-bottom of the Levant, where it forms a stratum that is continually undergoing a slow but steady increase in thickness, the microscopic researches of Professor Williamson have shown that not only are there multitudes of minute remains of living organisms, both animal and vegetable, but that it is entirely or almost wholly composed of such remains." [Illustration: ORGANISMS FROM MUD.] The water of the sea is everywhere salt, except at the mouths of great rivers, where the quantity of fresh water displaces that which properly belongs to the sea. The cause of its saltness is the solution of a natural mineral (chloride of sodium) which exists in the earth in great abundance in layers of a crystalline structure, and as this chloride of sodium (common salt) is soluble in water, of course it is all dissolved by the water in whatever situation they may come into contact with each other. The composition of sea-water differs slightly in different parts of the earth, the southern seas being slightly more salt than the northern. As a general rule, about five per cent. of solid matter is contained in sea-water, of this rather more than half (5.7) is chloride of sodium; the greater part of the other half consists of different salts of magnesia, and this is the whole source of the medicines known as Epsom salts and magnesia, the former being the sulphate and the latter the carbonate of magnesia. Iodine, another article used in medicine and photography, is also extracted from sea-water, which however contains but a very minute portion, too small to be detected as a general rule, but extracted by burning certain sea-weeds, in the ashes of which it is found in sufficient quantity to be separated.