In the first section of this book we made a study of the processes and agencies which are still at work upon and within the earth, tending to modify it in one or other particular. We there found that slow but ceaseless cycles of change take place and that a continual circulation of material is going on.

We have now to take up the second branch of our subject, that of structural geology, which deals with the materials of the earth's crust, their mode of occurrence, and their arrangement into great masses. Structural geology is, however, not merely a descriptive study; hand in hand with the examination of the rock-masses must go the attempt to explain their structure, and to show how they have come to be as we find them. Dynamical principles must be continually called in to interpret the facts of structure, and many of the observations concerning the construction, destruction, and reconstruction of rocks find their application in the study of structure.

This application cannot, in all cases, be made with confidence, both because a given structure may often be referred, with equal probability, to different processes, and because certain of the great dynamical agencies are so slow and gradual in their mode of operation, that no one has ever been able to observe them at work. In this latter class of cases the agency must be inferred, not from anything which we have actually seen accomplished, but from the traces which it has left in the structure. Under such circumstances, it need not surprise us to find that the explanation is not always obvious, but may be very problematical, and that great differences of opinion may arise concerning the rightful interpretation of a complex region.

Here, as in all other provinces of geology, the historical standpoint is the dominant one. Our object is to learn, not only the agencies which have produced the structures and the way in which they operated, but also the successive steps by which the structures originated, the order in which they occurred, and their geological date. Thus they may be coordinated into the great history of the earth, which it is the main problem of geology to construct.