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Show XX GEOLOGY OF THE HIGH PLATEAUS. hundreds of thin sections for microscopic investigation have long since been made. It was intended, also, to describe these rocks thoroughly and illustrate the microscopic characters with a large collection of colored plates. But the contemplated work was too costly for the very limited appropriation at the disposal of Professor Powell. A considerable number of chemical analyses have been made by myself, but petrographers have very properly adopted the habit of relying upon other parties to furnish their chemical analyses, and I have therefore omitted to publish them. My conviction is that the chemical analysis of volcanic rocks should, whenever practicable, accompany the description of microscopic characters, for it seems to me that the two lines of investigation are mutually dependent. It is hoped that at no distant day the contemplated work may be brought to completion in a supplementary volume, for the want of it is most deeply felt in presenting the present one. THE ATLAS. The atlas which accompanies this work has been prepared with great care. The first double sheet represents by contours the topography of the country. The primary triangulation is by Prof A. H. Thompson, and the topographical work by Messrs. J. II. Renshawe and Walter H. Graves, under Professor Thompson's supervision. Having been in immediate contact with these gentlemen during much of the time occupied by their field work, and having familiarized myself with their methods, I can testify to the great care and accuracy with which that work has been performed. The detail work has been done with plane-tables upon sheets on which the primary and secondary triangulations had been accurately plotted. These sheets were carefully filled up with details in the field, and when they were brought back to Washington contained the material which was used in the preparation of the final map. Whatever could be sighted from the stations occupied has been located by triangulation and plane-table sights and not by sketching. Messrs. Renshawe and Graves acquired great skill in the use of the plane-table, and worked with surprising accuracy and rapidity. Each of them covered more than 2,000 square miles in a season. The geological map has been colored by myself. The northern half of the sheet is for the most part held to be accurate in details. In the |