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Show AUTHOR'S PEEFACE. xxiii value of it is the facility it affords the student of testing the accuracy of his work. He cannot commit a serious error in making his stereogram without knowing it. He cannot proceed far in his work without becoming conscious of the defects and gaps in his knowledge, and, best of all, he obtains an index pointing to the very localities which he must revisit in order to supplement the deficiencies. A stereogram is a laborious work, but it abundantly repays the labor expended upon it. The writer who achieves one will'know the structure of the objects he is describing in a way and with a thoroughness he could never hope for from any other means. Unfortunately this method of systematizing observation is of very limited applicability. Much disturbed regions and countries which have preserved very obscurely the records of their displacements are hardly capable of such a discussion. The stereogram cannot take the place of the ordinary geological sections, though it can embody in one illustration some of the most important features of a hundred or more. It is my pleasant duty to acknowledge the obligations which I owe to Professor Powell for the earnest support he has given me during the work of exploration and while the report has been in process of preparation. Every facility which he could supply has been placed at my disposal, whether in the field or in the office. But the greatest debt which I owe him is for the scientific advice and assistance he has given me. He has been not merely the director and administrator of his survey, but in the most literal sense its chief geologist. During the period of his field work in the Plateau Country (from 1869 to 1874) he had mastered with great rapidity and acumen the broader facts and had co-ordinated them into a system which was novel in many respects and which further research has proved to be perfectly sound. The geological phenomena encountered in that region are indeed governed by the same fundamental laws which prevail elsewhere, but the conditions under which those laws operate are altogether novel and peculiar, and the results which they produce are so singular that they seem at first anomalous and then mysterious. The geologist who is skilled in the conventional methods of investigation, the older applications of prin ciples, and the routine logic which have long been in vogue, might well have been excused if he had found in this strange land little else than |