Geology is the study of complex natural experiments conducted on a large scale in both time and space. The experiments are neither reversible nor repeatable. They cannot be directly observed; but they must be reconstructed historically.

We view samples of the accessible universe as a library of experiments that have already been done (at different times and sites). Our art must be to select those materials which have a persistent memory and which together can bring testimony to the natural experiment of interest.

It is the nature of the history of the earth that a geologist has available to him only partial information. Occasional lines from disconnected paragraphs in obscuranist chapters are what can be read. Violence in the handling of the book through time has caused many of these chapters to be ripped and reassembled out of context. That the gist of early chapters can be deciphered at all is a credit to the perseverance and imagination not always associated with other sciences. The geologist operates at all times in an environment characterized by a high degree of uncertainty and ornamented with end products which are the outcomes of the interactions of many complex variables. He sees only the end, and has to induce the processes and responses that filled the time since the beginning.