24 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
f 
particularly abundant in the vicinity of the contact between the two granites south¬ 
west of Guyot Hill. Cross and Mathews, from their studies in the Pikes Peak 
quadrangle, reached the same conclusion with regard to the relative age of these 
granites. 
Before the Tertiary eruptions broke through the plateau the principal mass of 
Cripple Creek granite probably extended eastward to West Beaver Creek, for the 
granite between Bull and Ironclad liills and some masses near Cameron are of this 
type. In the southern part of the district, near Victor, are some small bodies of 
similar granite apparently intrusive in the Pikes Peak granite. 
The various intrusive contacts between the pre-Cambrian rocks are unaccom¬ 
panied by any important mineralization, even pvrite being notably lacking in these 
rocks, and the prevolcanic structure of the ancient crystalline plateau is of economic 
importance only in so far as it affects the distribution of the different rock types in 
the rim of the volcanic conduit . Practical^ no deposits of value occur, for example, 
in schist, but some of the most productive ore bodies in the district have been found 
in the Pikes Peak granite close to the irruptive contact. The Cripple Creek granite, 
moreover, where it forms a part of the rim has proved less productive than the 
gneiss. It is thus probable that the economic history of the district would have 
been different had not the prevolcanic structure determined that a large part of the 
volcanic neck, particularly between Guyot Hill and Goldfield, should be bordered 
by the Pikes Peak granite. It has already been pointed out that the principal 
volcanic eruptions broke through the pre-Cambrian rocks near the point of a wedge¬ 
like projection of gneiss, schist, Cripple Creek granite, Spring Creek granite, and 
olivine syenite into the Pikes Peak granite. It is possible that this was not a mere 
coincidence, but that the pre-Cambrian structure as revealed in this areal distri¬ 
bution of the rocks was an important factor in determining the point of eruption. 
The granites and gneiss of the Cripple Creek district are cut by a number of 
diabase dikes that are older than the Tertiary eruptives and that may belong to the 
same eruptive series as the olivine syenite and anorthosite of Iron Mountain. So 
far as known, these dikes have no economic importance. 
FORM OF THE VOLCANIC NECK. 
It is essential to a clear understanding of the mutual relations of the Cripple 
Creek rocks to remember that the central part of the district is occupied by a 
volcanic neck similar in some respects to those described by Geikie in the south of 
Scotland. At the time of eruptive activity the region seems to have been a granitic 
plateau not differing greatly in the general character of its relief nor even in actual 
surface from the plateau of to-day. The first manifestation of volcanism of which 
there is any record was the formation, by one or more tremendous explosions, of a 
great chasm in this plateau. The granite above the volcanic hearth was shattered 
and blown into the air, falling back as fine fragments partly into the pit formed 
by the explosion and partly over the surface of the plateau. Eruptions of phono- 
lite, latite-phonolite, and syenite followed, but do not appear to have issued in a 
simple sequence. Successive eruptive paroxysms probably shattered and com¬ 
minuted the products of earlier outbursts. The breccia now occupying the greater 
part of the volcanic throat exhibits in many places such a thorough mingling of 
