PREFACE, 
Field work in the Aspen district was begun by the writer and Mr. 
Tower about July 1, 1895, and continued to about December 1 of the same 
year. ‘The summer months were chiefly spent in the study of the surface, 
while the fall and early winter were devoted to the investigation of under- 
ground phenomena, as shown in mine workings. 
At the beginning of the work the first reconnaissance showed that the 
structure of the district was peculiarly complicated, and it was recognized 
that in order to finish the imvestigation within a reasonable period of time 
the methods of work must be modified in proportion to the degree of 
complication. Work was begun in the most complicated district—that 
represented in the northern part of the Tourtelotte Park special map (Atlas 
Sheet XII). Here at first a system of cross-sectioning at intervals of a few 
hundred feet was tried, but it was found that the complication was such 
that no accurate results could be obtained. The system was then adopted 
of examining the whole ground thoroughly by following continuously 
formation and fault lines where these outcropped, and locating accurately 
every place where bed rock could be found. These results were plotted 
together on a topographical map as a record of fact to which all final solu- 
tions must conform, and a preliminary complete working map, which filled 
out by inference and estimation what the record of fact did not furnish, 
was constructed. ‘This being finished, special maps covering the important 
mining districts were constructed on a much larger scale. In these districts 
the mines were carefully gone through, the mine maps obtained, reduced 
_ to a common scale, and plotted together on the topographical maps. When 
all the data which could be obtained from this underground work had been 
collected, they were combined with the information already obtained from 
the surface, and maps on the 300-foot scale were constructed, the details 
being worked out by a system of cross-sectioning preliminary to the final 
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