OCR Text |
Show xxii GEOLOGY OF THE HIGH PLATEAUS. lavas and the conglomerates would obviously be impossible. With the foregoing exceptions the distribution of the strata is given with great confidence In the exceptional cases the errors are believed to be so small as not to sensibly impair the accuracy of the map. The relief map was prepared in the following manner: A plaster cast about five feet square was made, the horizontal and vertical scale being the same. The data for the cast were obtained from the contour map. The cast was then photographed, and a copy of the photograph was drawn upon stone. The map (Sheet No. 4), showing the arrangement of the faults and flexures, was designed to show at a glance the connection, relations, and in some cases the continuity of the greater structure lines of the High Plateaus with those of the Kaibab district around the Grand Cailon of the Colorado. The Kaibab or Grand' Canon faults have been already worked out in an admirable manner by Powell. The importance of connecting the two districts by these common features is very great, and is not only essential to the present work, but will have, if possible, still greater importance when the geology of the southwestern part of the Plateau Province is discussed. Only the greater displacements are here given. There are very many smaller ones which are not so well known nor so well identified. Those which are given have been traced rigorously mile by mile so far as they are represented, excepting, however, the portions which extend south of the Colorado. The course of these faults south of the Grand Canon has been given to me by Mr. Gr. K. Gilbert, who has in part identified their existence in that region, though I presume that he would not wish to be understood as attaching a very high degree of accuracy to his designations, having made merely a preliminary reconnaissance in that region. The stereogram has been worked out with great care. It is the consolidated expression of a very large number of sections made in the field, together with the results obtained by tracing continuously each fault along its -course. This mode of illustrating displacements is by no means all that could be desired and has some serious defects But it seems to be a great improvement in the means of illustrating structure, since it groups the dominant features together in their proper relations. Probably the greatest |