CHAPTER II.—GENERAL GEOLOGY. 
INTRODUCTORY STATEMENT. 
In view of the well-known work of Mr. Cross, it is not necessary,' as in the 
earlier report, to present a detailed account of the geology of the district. The 
general relations of the rocks and the main facts of the volcanic historj" are now 
familiar to those interested in the region, and the first official report is still a 
valuable source of detailed and accurate descriptions of particular localities. The 
plan adopted in the present work, therefore, does not contemplate so exhaustive 
a description of the district as would be appropriate in a new geological field. It 
is proposed to recount briefly the essential results obtained by Mr. Cross and to 
indicate in a few words the general character of the modification of these results 
required by later study. Mr. Graton’s report on the petrography of the Cripple 
Creek rocks, presenting further details of the changes involved, forms a separate 
chapter and is followed by a sketch of the geological structure of the district with 
such additional matter as is necessary to supply a satisfactory geological basis 
for the discussion of the ore deposits. 
The new geological map of the district forms PI. II and is accompanied by 
five sections, on which full black lines indicate that the contact lines are actually 
known from mining developments. 
FIRST GEOLOGICAL SURVEY OF THE OISTRICT. 
When Mr. Cross made his careful study of the geology of the Cripple Creek 
district ten years ago, mining had barely begun and the various hills were not, 
as now, perforated by deep underground workings. That his work has in general 
stood the test of subsequent mining exploration and continues to be highly regarded 
in the district is convincing testimony to its excellence. Later workers, however 
they may amplify or modify his results, should fairly acknowledge their debt to 
the pioneer who first unraveled the structure and deciphered the historj' of the 
Cripple Creek volcanic complex. The account of the district as given by Cross 
may be very briefly summarized as follows: 
The Cripple Creek hills lie near the eastern border of a lofty and deeply dis¬ 
sected plateau which slopes gently westward for 40 miles, from the southern end 
of the Colorado Range, dominated by Pikes Peak, to the relatively low hills con¬ 
necting the Mosquito and Sangre de Cristo ranges. The prevailing rocks of this 
plateau are granites, gneisses, and schists. The granites inclose masses of Algonkian 
quartzite and are therefore post-Archean, but they are older than the only Cambrian 
sediments known in Colorado. During Tertiary time volcanic eruptions broke 
through these ancient rocks at several points and piled tuffs, breccias, and lavas 
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