3. The beds are generally homogeneous; that is to say, that each one is formed of a similar substance, or of diverse substances which have co-existed, and have formed a compact whole. The line of separation isolating them from one another is always distinctly defined as in the different parts of a ship. No part is seen mingled or lost in another, each remains within its own respective limits. Such is the case, for example, in the colors of the prism or the rainbow.
By these characters, observers decided that they had been successively formed, deposited upon one another by different causes and conditions. The deepest have naturally been formed first, and those nearest the surface subsequently. The last of all, that which is found on the surface, is the bed of vegetable which owes its properties to the destruction of organic matter which produces plants and animals.