XX GEOLOGY OF THE BTGH 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 slieets 

 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 



