56 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
DIABASE. 
Numerous dikes and small masses of finely granular, dark rocks cut the gran¬ 
ites, gneisses, and schists, but are not intrusive into the later volcanic rocks of the 
district. This difference serves to distinguish them from the basaltic rocks of 
somewhat similar appearance which occur principally in the younger volcanics. 
These older rocks are usually decomposed at the surface into a greenish mass with 
characteristic concentric disintegration, but at no great depth fairly fresh rock 
is usually encountered. They occur principally in and northwest of the town of 
Cripple Creek and along the northern slopes of Brind and Straub mountains. 
Cross assigned them to the Algonkian or early Cambrian. 
Feldspar is developed in laths parallel to the brachy-axis which are frequently 
twinned according to the Carlsbad and pericline laws, in addition to the common 
albite twinning. It is usually turbid through kaolinization. The composition of 
the greater part corresponds to calcic labradorite, but in several instances there 
occur narrow outer zones of a clearer, more alkalic variety, which is sometimes 
even ortlioclase. Pyroxene is present in considerable amount and seems to be 
diallage. It usually shows a slight pleochroism in purple, and doubtless carries a 
small percentage of titanium. A common decomposition product is a brownish- 
green fibrous amphibole corresponding to uralite. In other cases the pyroxene 
alters into a brownish-yellow aggregate of micaceous aspect. Olivine itself was 
not seen, but the freshest specimens hold patches of secondary material, built of 
fibers arranged in the characteristic net-like manner and possessing the outline of 
olivine. This secondary material is yellow or brown, somewhat pleochroic, and 
gives high polarization colors like those of muscovite. Carbonates and a little 
serpentine are also present in some of these areas. Numerous grains of deep 
reddish-brown biotite of strong pleochroism, or in a few cases ordinary brown 
biotite, occur in irregular flakes. Apatite and magnetite are usually plentiful as 
accessories, the latter mineral often being present in the grill-like skeleton crystals 
frequently seen in rocks of this character. Carbonates, sericite, and abundant 
epidote and limonite are formed on the decomposition of these rocks. . 
The laths of feldspar are arranged among the other constituents in the man¬ 
ner known as the opliitic texture, which is characteristic of the dolerites or diabases. 
It is apparent that these rocks possess many resemblances to the doleritic 
contact facies of the olivine-syenite mass. The character of both monoclinic and 
triclinic feldspar and of the pyroxene and the presence of olivine and red mica 
relate the two rocks very closely and make it probable that many if not all of the 
diabase dikes of the Cripple Creek district are connected with the olivine-syenite 
intrusion. One or two dikes which contain ordinary biotite and in which olivine 
was not detected may be of different age. 
TERTIARY VOLCANIC ROCKS. 
The younger group comprises mainly the rocks of the Cripple Creek volcano. 
These are phonolite, leucitophyre (?), latite-phonolite, trachyte, syenite, trachydo- 
lerite, vogesite, monchiquite, and volcanic breccia. In this group is also included 
an areally unimportant occurrence of rhyolite, which had a different source, but 
