42 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
report is not only based on the studies of the writer, but also embodies those results 
of the extended investigations of Mr. Lindgren and Mr. Ransome which have a 
bearing on petrography. 
Under these advantageous conditions the w r ork w r as carried on so far as possible 
independently and as if original, and it is gratifying to know that it sustains the 
main conclusions of Mr. Cross. On the other hand, results which differ from his 
have been obtained in some instances and considerable that is new has come to light. 
Such differences, however, are in many cases unessential and may often be explained 
by the difference in conditions of w r ork already noted. 
While the present account of the petrology of the district thus comprises much 
that is contained in the earlier report, it has been deemed advisable to present it in 
full. This decision has been largely based on the fact that the early report is out 
of print and also on the belief that this independent verification of Cross’s principal 
results can not fail to add to then* value. Moreover, an adequate presentation of 
such features as are new and different can hardly be given without a full statement 
of the results common to both investigations. 
The essentially economic character of the study of which the petrology is but a 
part has rendered it desirable that the conclusions reached and the distinctions 
recognized in the mapping and in the petrographic descriptions be of practical 
application not only by the experienced petrographer but by the miner as well. 
Minor variations within rock masses or among different bodies of the same general 
type have consequently not been made prominent, though they have been noted 
and described. While this desideratum has been borne in mind, it has, neverthe¬ 
less, been the endeavor to make the treatment scientifically accurate and to bring 
out such petrologic principles as are exemplified by the rocks. It may not be inap¬ 
propriate to state that the descriptions and conclusions here given are based on a 
study of the large collections of rocks, with numerous thin sections, from the Pikes 
Peak quadrangle and the Cripple Creek district made by Mr. Cross and kindly placed 
by him at the writer’s disposal, and also of the specimens, numbering about 750, 
with very many thin sections, collected during the resurvev of the district. Each 
rock type is thus very well represented, and nearly all the important rock, masses 
have been studied by means of numerous specimens. It has been sought throughout 
to make this a study of geological units—rock masses—instead, as is so often the 
case, of particular, individual hand specimens. It is believed that the large number 
of specimens lias made possible a fairly satisfactory achievement of this aim. 
In view of the close relation existing between the processes of rock alteration 
and those of ore deposition, especial attention has been given to the subject of 
decomposition and alteration of the mineral components of the rocks. 
OUTLINE OF THE ROCK FORMATIONS. 
The rock formations of the Cripple Creek district may be divided into two very 
distinct groups. One comprises ancient crystalline rocks, which make up a great 
portion of the Colorado range and the plateau to the west and upon which Paleozoic 
and later sediments have been deposited. These rocks originally occupied the 
entire area of the district. 
