INTRODUCTION. ROXEX 
opportunity in their daily work to subject the maps to the severest tests, 
have borne testimony to their high degree of accuracy. The geological 
examination of the district, which was planned for the summer of 1892, was 
necessarily postponed in consequence of the provisions in the appropriation 
act passed by Congress in July, 1892, which not only cut down the amount 
allotted to geological work, but specifically reduced the number of geolo- 
gists employed, and resulted in the discharge from the Survey of the four 
geologists who were especially devoted to economic work, of whom the 
writer was one. 
The present Director assumed control of the work of the Survey in 
July, 1894, but it was then too late to undertake so elaborate a piece of 
work as the Aspen survey, and it was therefore postponed until the season 
of 1895. 
In consequence of a severe attack of pneumonia in the early part of 
that season, the writer was incapacitated for the arduous physical labor 
involved in the work planned, and was obliged to trust the practical 
execution of the work to Mr. J. E. Spurr, who had been his assistant dur- 
ing the previous season, and to content himself with acting in an advisory 
capacity and making two short visits to the district during its progress. 
Mr. Spurr was occupied in the field work from June to December, 
1895, and was assisted during this time by Mr. G. W. Tower. Too great 
credit can not be given to these two geologists for the zeal and energy 
which they have displayed in unraveling this most difficult problem in 
structural and economic geology in so short a time and in so thorough a 
manner. The magnitude of the work will be appreciated by an exami- 
nation of the following pages and the atlas of maps and sections which 
accompanies the volume. 
Having received orders to proceed to Alaska for an examination of 
the interior regions in the valley of the Yukon, Mr. Spurr has been obliged 
to complete his office studies of the material gathered, and the platting of 
the geological data on maps and sections, as well as the writing of the 
text, so as to leave for his new field of work by June 1. It has been 
thought best to publish the volume at once, even though, owing to the 
great pressure under which it has been written, there may have been less 
attention given to the form of presentation than if it had been more delib- 
erately considered. For the facts, as well as for the theoretical conclusions 
presented, Mr. Spurr desires to assume the entire responsibility. 
