• What happened? Earthquakes? Floods? Drought? Volcanic eruptions? Meteorite impacts? Ice ages? Emergence of life? Extinction of species? Appearance of continents? Invasion and retreat of seas?
• When did it happen? When did the earth come into being? Acquire a moon? An atmosphere? An ocean? When did life first adorn its surface? When did insects, fish, mammals, birds, dinosaurs appear? Disappear?
• How quickly did it happen? Do mountains pop up and vanish in a flash? Did the dinosaurs vanish with a whimper or a bang? Did ice sweep over the earth's surface or advance slowly, inch by inch?

Almost all the earth's history, to the extent that it is recorded, is recorded in rocks. The most recent events may leave their imprints in human artifacts, myth, written history, the growth rings of trees, the body and structure of other living organisms. But the usefulness of such receptacles of history rapidly fades to zero as the journey into the past becomes serious in its ambition.

Part 1 of 'The Fourth Dimension' examined ways in which an understanding of earth processes can be extracted from rocks. Composition, texture, structure and field relationships reveal the circumstances of rock formation: what the environment was like and was going on at a particular place at a particular time.

A process not only takes place at a particular place and time but also through time. In understanding process - for example, the creation, transport, deposition and lithification of sediment - a small, local slice of the earth's history is unraveled. In turn, the working out of history sheds light on process: determining what came first, what second, and so on, places limits on causal relationships. Consider the mechanism of dinosaur extinction. If it can be shown that the last dinosaur died before a certain meteorite impact occured, the process that caused dinosaur extinction cannot have involved that meteorite impact.

It is important to recall the assumptions underlying the geologists' approach to history.

This is what geologists assume:

• Events in earth history leave decipherable physical evidence of their occurrence in rocks. Indeed, the characteristics that rocks display are related in an orderly fashion to the processes that created the rocks rather than being randomly stamped upon them.
• The orderliness to the relationship between processes and their physical results provides a key to deciphering earth history. This key exists because past events obeyed the same physical laws that govern the universe today.
• Thus, studying the way the earth works now furnishes an understanding of the characteristics of rocks and the history they represent: past rock-forming processes may be identified, the environments in which they operated may be determined, and the sequence in which they took place may be unraveled. In short, the present is the key to the past.
• An understanding of the present may be gained by direct observation of nature and also by careful analogy between experiment and nature.
• Prediction is reliable in principle because the physical laws that govern the universe today are independent of time and will continue unchanged into the future.