AUTUOR'S PREFACE. XXUl 



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. Unfor- 

 tunately this method of systematizing observation is of very limited appli- 

 cability. 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 geologi- 

 cal 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 lie 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 apphcations 



of principles, 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 



