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Rocks of pure flint, pure clay, or pure lime, are rarely or never met with. Most rocks are made up of several different substances melted together. In the fire-built rocks no remains of animals are found, though in water-built rocks they abound.
Observing and reasoning thus, geologists have drawn up a general plan or order of strata; and the whole of the vast masses of water-built rocks throughout the world have been arranged in a regular succession of classes, rising step by step from earliest ages up to the present time.
We find rocks piled upon rocks in a certain order, so that we may generally be pretty confident that the lower rocks were first made, and the upper rocks the latest built. Further than this, we find in all the said layers of water-built rocks signs of past life. As already stated, much of this life was ocean-life, though not all.
Blue and green and gold is the world, and the little brown boys play about their water-built villages, tumbling in and out of the water, and living in the warm sea as much as on land day by day. Shoals of them come round us in their catamarans and dive for money, catching the silver bit as it twinkles down through the water, even though they make their spring from many yards off.
And geologists are now able to state with tolerable confidence that, however old many of the granites may be, yet a large amount of the fire-built rocks are no older than the water-built rocks which lie over them. So by many geologists the names of Primary, Transition, and Secondary Formations are pretty well given up. But if they really do lie under, how can they possibly be of the same age?
Water-built rocks are sometimes divided into two classes those which only contain occasional animal remains, and those which are more or less built up of the skeletons of animals. There are some exceedingly tiny creatures inhabiting the ocean, called Rhizopods.
So the name of Primary Rocks, or First Rocks, was given to the granites and other such rocks, and the name of Secondary Rocks to all water-built rocks; while those of the third class were called Transition Rocks, because they seemed to be a kind of link or stepping-stone in the change from the First to the Second Rocks.
Before seeking to describe the diverse colors made largely by one substance, let us remember that while silica, the principal part of these water-built mounds, is one of the three parts of granite, namely, the white crystal quartz, it is also the substance of the beautifully variegated jasper, the lapis lazuli, the green malachite, and the opal, with its cloudy milk-whiteness through which flashes its heart of fire.
The chief reason for the general belief that fire-built rocks were older than water-built ones was, that the former are as a rule found to lie lower than the latter. They form, as it were, the basement of the building, while the top-stories are made of water-built rocks. Many still believe that there is much truth in the thought.
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