As the shells are tiny and uniform in size and have a hardness of 6, the diatomaceous earth is used to make a great variety of polishes, scouring soaps, tooth paste, as a filler in certain kinds of paper, in making waterglass, as an absorbent for nitroglycerine, and as packing in insulating compounds, where asbestos would otherwise be used. Deposits of freshwater diatoms are found all over the United States, usually in thin layers of limited extent, especially in Massachusetts, New York, Michigan, etc. The marine deposits of diatoms are on a much larger scale, there being beds of diatoms in Anne Arundel, Calvert and Charles Counties, Md., up to 25 or 30 feet in thickness. In Santa Barbara County, Cal., there is one bed 2400 feet thick and another 4700 feet thick, beside many other smaller ones. The enormous former wealth of life indicated by these great deposits may be suggested, when it is remembered that it takes about 120,000,000 to make an ounce in weight. They reproduce on an average about once in five days, so that from a single diatom the offspring possible under favorable conditions would amount to over 16,000,000 in four months or over 60 tons in a year. Of such an order is the potential increase of animals or plants, no matter how small, if the rate of reproduction is high. Metamorphic Rocks Either a sedimentary or an igneous rock, which has been altered by the combined activities of heat, pressure and chemical action, becomes a metamorphic rock. The process is essentially one, during which the layers of rock come under the influence of such temperatures as are associated with the formation of granite or lavas. Such material as is actually melted becomes igneous rock, but adjacent to the masses actually melted are other rocks which do not melt but, according to the temperature, are more or less changed, and these are the metamorphic rocks. At a distance from the molten masses the changes are minor, but close to the molten magmas extensive changes take place. Though not actually melted the rock near the heat center may be softened, usually is, in which case pebbles and grains or even crystals become soft and plastic, and, as a result of the great pressure, are flattened, giving the rock, when it cools again, a striated appearance. At these high temperatures the water in the rock and also some other substances vaporize, and the hot steam and vapor are active agents in making a great many chemical changes. In some cases material like clay is changed into micas, or chlorite, etc.; in other cases the elements of a mineral will be segregated and large crystals will appear scattered through the metamorphic rock, such as garnets, staurolites, etc.