GEOLOGICAL STRUCTURE OF THE DISTRICT. 
23 
that the material was thrown into the air by an explosive eruption and fell thickly 
upon the surrounding plateau, to be in some places partially reworked and distrib¬ 
uted by streams. 
AGE OF THE ERUPTIONS. 
The fact that the grits of Grouse Hill and Straub Mountain can not be correlated 
with the High Park lake beds does not involve any great change in the assignment 
of a probable geological date to the Cripple Creek eruptions. Inasmuch as the 
accumulation of the grits probabty marked the beginning of local volcanic activity, 
and as both grits and rhyolite are intruded by phonolite from the Cripple Creek 
center, it follows that the Miocene (or at least post-Oligocene) rhyolite was erupted 
within the period during which the Cripple Creek volcano was active. 
GEOLOGICAL STRUCTURE OF TIIE DISTRICT. 
CRYSTALLINE ROCKS OF THE PREVOLCANIC PLATEAU. 
As appears from the accompanying map (PI. II), the schist and gneiss occur 
chiefly in the northwest corner of the district. The two rocks are in irregular and 
intimate association and constitute a thoroughly metamorphosed complex of 
unknown derivation. It is probable, however, that the gneiss was originally a 
granitic rock and there is some evidence to support the view that the schist, which 
at the present time is composed of muscovite, quartz, fibrolite, and magnetite, is 
an extremely metamorphosed sediment. The two rocks exhibit intermediate facies 
and can not everywhere be distinguished. This metamorphism was probably 
effected in pre-Cambrian time and long antedates the eruptions from the Cripple 
Creek volcanic center. 
The olivine syenite and Spring Creek granite occur in the extreme northwest 
corner of the district and are intrusive in the gneiss and schist. 
The Pikes Peak granite is the most extensively developed formation in the 
district. It is prevalent on the north, east, and south sides of the central volcanic 
area and is the principal rock of the plateau country for miles in these directions. 
It is intrusive into the gneiss and schist, and probably also into the Spring Creek 
granite and the olivine syenite. 
The Cripple Creek granite occupies a considerable area in the western part of t-he 
district, west and southwest of the town of Cripple Creek. It is clearly intrusive 
into the gneiss and schist and has invaded these rocks irregularly, not only in large 
masses, but in numerous dikes, as may be well seen at many points in the southern 
part of Cripple Creek town. The reddish color of the younger rock, contrasting 
with the gray tint of the gneiss and schist, renders these dikes readily recognizable. 
It is probably intrusive also into the Pikes Peak granite; for though no decisive 
evidence bearing upon this point was found within the district, yet the occurrence 
in the Pikes Peak granite of fine-grained granitic dikes which are similar in litho¬ 
logical character to the Cripple Creek granite, the absence of gneissic structure in 
the latter and its frequency in the Pikes Peak granite, particularly near the contact 
of the two rocks, are indicative of this relation. Shear zones, along which the 
sheared granite is often metamorphosed to schist, occur in the Pikes Peak granite, 
but not, so far as observed, in the Cripple Creek granite. Such schist bands are 
