So far we have talked only about rocks that are of the Cambrian Epoch or younger—rocks that may contain fossils. Yet there are vast areas (most of Canada, for example) that are covered with rocks older than the Cambrian formation. Some Precambrian fossils have been found, but they are so rare that they are useless for dating the strata containing them. Long-range correlation of Precambrian rocks must rely on nuclear measurements. Therefore it has been only in the last dozen or so years that some order could be established for the Precambrian rock sequences. The elaborate Precambrian stratigraphies (arrangements of strata in sequence) proposed in the past, most of them based on superficial similarities of the rocks in one place to those in another place, now have been drastically altered and in some cases completely overturned by nuclear measurements. We are still far from understanding the sequence of all the events in that vast span of time we call the Precambrian. Many thousands of nuclear age determinations will have to be made to lighten the dark corners of our ignorance. [Illustration: _Folded strata of Precambrian rocks, including limestones and shales, in Glacier National Park, Montana._] AND WHERE DO WE GO FROM HERE? Perhaps we must first realize that we really haven’t come very far yet. Granted that the age of rocks in many parts of the world is now suddenly known—and that this was a total mystery some dozen years ago. Granted that enormous strides forward have been made. It’s only a beginning. Vast areas of the world are still geologically unexplored. The geologic time scale is still fragmentary and crude. Thousands of important geologic questions remain to be defined, explored, and answered by nuclear age determination. And—as is inevitable in science—many of them will lead to new questions. It is apparent that dating techniques have barely begun to be used and understood by geologists. But apart from geologic work, what else is in store? It is difficult to predict, but probably the most important advance in the next decade or two will come when we obtain samples of rock from the moon. Will there be young rocks there or will they all be 4550 million years old? Or will they perhaps be some other age? The chemical composition and nuclear ages of the first moon samples will probably be the most important information we can hope to obtain from them. These results from the moon will contribute enormously to our understanding of the processes that formed the earth, made the continents, and determined the major features of our world. We have a long way to go. GLOSSARY AEON One billion (10⁹) years. ALPHA DECAY Radioactive decay with emission of an alpha particle. ALPHA PARTICLE Essentially the nucleus of helium, composed of two neutrons and two protons with double positive charge.