CHAPTER III.—DESCRIPTION AND PETROLOGY OF THE META- 
MORPH1C AND IGNEOUS ROCKS. 
By L. C. Graton. 
INTRODUCTION’.« 
PLAN AND SCOPE OF THE INVESTIGATION. 
The rocks of the Cripple Creek district were made the subject of an extended 
petrographic study by Mr. Whitman Cross in connection with his examination of 
the geology of the region in 1894. At that time mining developments in the district 
were only well begun, and the geological worker had in most cases to be content 
with such information as could be derived from natural exposures. For various 
reasons—forest covering, presence of soil and slide, and in many cases profound 
decomposition of the rocks—the data thus obtained were in large part meager and 
unsatisfactory. Decomposition caused most trouble in the most critical portion of 
the district—the volcanic area—and made particularly difficult of attainment a 
good knowledge of the breccia and the basic dike rocks. In spite of these facts, 
that portion of Mr. Cross’s report 6 which deals with the description of the rock 
formations not only gave the first insight into the nature and relations of these 
extremely interesting rocks, but furnished an excellent conception, broad and at 
the same time detailed, of the petrology of this district. His work is so well known 
to petrographers that further comment is hardly necessary. 
The present investigation was conducted under much more favorable circum¬ 
stances. That portion of the district which is of greatest petrographic importance 
is now prospected to a degree probably unequaled elsewhere. Developments rang¬ 
ing from shallow pits and trenches to very extensive mine workings actually honey¬ 
comb the rocks near the surface. These openings in several ways lend aid to the 
study of the geology and petrology. They furnish definite, satisfactory exposures 
of the rocks; they enable fresh material to be collected almost at will, and the 
deeper workings, reaching in some cases to a depth of 1,500 feet, afford an under¬ 
standing of structural relations which no amount of surface study could give. With 
the conclusions reached by Cross already in hand, more attention could be given 
to details in this later work. Furthermore, a somewhat greater length of time was 
available for this examination than was at the disposal of Mr. Cross. Finally, this 
a The writer wishes to acknowledge his obligation to the authors of this report for their unfailing interest and cooperation 
in his work. He owes much to Mr. Ransome for suggestions and helpful criticism both in the field and in the office. The 
larger part of the office work has been done under the direct supervision Of Mr. Lindgren, for whose advice, criticism, and 
aid he is deeply indebted. To both these gentlemen are due his thanks for the opportunity to study and describe this most 
interesting group of rocks. The value of this chapter has been much increased by a careful criticism by Mr. Cross. 
b Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, pp. 20-58. 
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