SYNOPSIS OF PART I. 
CHAPTER I.—INTRODUCTION. 
The Cripple Creek gold deposits, discovered in 1891, were investigated by Messrs. 
Cross and Penrose, of the United States Geological Survey, in 1894. The present 
reexamination was requested by citizens of Colorado, and has been carried out 
under the financial cooperation of the State with the Federal Survey. It has 
involved complete revision of the topographic map of the district used as a base by 
Cross and Penrose, the running of a line of accurate levels from Colorado Springs to 
Cripple Creek, remapping of the geology, and a thorough stud}^ of the extensive 
mine workings opened during the past ten years. Due acknowledgment is made of 
• the cordial assistance rendered by mining men, and a list is given of the important 
publications concerning the geology or mines of the district. 
CHAPTER II.—GENERAL GEOLOGY. 
Results of first geological survey of the district .—The Cripple Creek hills lie near 
the eastern border of a much dissected 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 connecting 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 upon the uneven surface of the plateau. The eruptive 
rocks of the Cripple Creek district are the products of one of the smaller isolated 
volcanic centers of this period, a center characterized by the eruption of phonolite, 
which does not occur elsewhere in this general region. 
The most voluminous products of the Cripple Creek volcano now preserved are 
tuffs and breccias. They occupy a rudely elliptical area in the center of the district, 
about 5 miles long in a northwest-southeast direction and about 3 miles wide. 
According to Cross, these breccias and tuffs rest in part upon an earlier flow of 
andesite, but mainly upon an unevenly eroded surface of the granites and schists. 
The breccia is much indurated and altered, but was thought by him to consist 
mainly of andesitic fragments. The most characteristic massive rock of the Cripple 
Creek volcano is phonolite. It occurs as dikes and masses in the breccia and in the 
surrounding granitic rocks. 
The general succession of igneous rocks, according to Cross, is as follows: The 
earliest rocks were andesites containing some orthoclase. Then came a series of 
allied phonolitic rocks, rich in alkalies and moderately rich in silica, together with 
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