Xvi PREFACE. 
areal mapping. From these smaller maps the structure was worked out by 
- degrees into the larger maps which inclosed them; and when all possible 
information had been assembled, final maps were constructed, ‘which, 
however, differed from the preliminary ones only in detail. 
The winter months were spent in working out more carefully the 
structure of the region, and in microscopic and comparative study of the 
rocks and ores. In the spring of 1896 the writer was unexpectedly 
requested to proceed to Alaska, and it became necessary to write this report 
in asomewhat hurried manner. It was not practicable, therefore, to devote 
much care to the writing, and the report is by no means so complete as it 
was originally intended to make it. Many things have been lightly passed 
over, or even omitted altogether, which might profitably have been worked 
out more elaborately. It is hoped, however, that the report may be of 
some use to the mining population, for whom it is chiefly intended, el 
that from a scientific point of view it may also possess some value. 
During all the field and office work on the Aspen district the writer 
has been ably seconded by Mr. George Warren Tower, to whose ability 
and energy much of the credit of accomplishing such a large amount of 
work in so short a time is due. 
In conclusion, thanks should be offered to the people of Aspen, who, 
with scarcely a single exception, have done everything in their power 
toward facilitating the work. To Mr. D W. Brunton and Mr. D. Rohlfing 
especial acknowledgments are due, but to name all the gentlemen from 
whom favors have been received would simply be to furnish a list of 
prominent mining men in Aspen; nor should the common miner and 
prospector be omitted, whose ready courtesy and hospitality will long be a 
pleasant remembrance. 
Jostan EDWARD SPURR. 
Wasuineton, May 20, 1896. 
