Algonkian quartzites are shown in the granitesand gneisses of this district, the most prominentones being in Cooper mountain, and in Eight Milecanyon at McCourt camp.
AREAS OF SEDIMENTARY ROCKS.
Southeastern area. —In the southeastern cornerof the district, where the sedimentary formationsof the plains are upturned at the base of thegranite mountain mass, the characteristic foot-hillstructure is very clearly illustrated. The outermostformation here exposed is the Dakota sandstoneforming mesas sloping gently to the south butpresenting a cliff front to the northwest andtoward Beaver creek, which cuts through it. Thegentle dip of the Dakota causes it to form mesasin the districts of the Colorado Springs, Pueblo and Canyon city sheets. The Morrison formationoccurs in a narro'w zone below the Dakota cliffand in the valley of Beaver creek. Between theMorrison and the base of the mountain is a broadzone underlain by the crumbling grits of the Foun-tain beds, which by their disintegration and readyerosion produce little park areas. The dip ofthese beds are 10° to 15°, except for the lowerstrata which acquire a dip of 30° or more, and runsome distance up on the foot-hill ridges. A rathernarrow band of the Harding and Manitou forma-tions appears below the Fountain grits, and ex-tends back on the ridges to variable distances.
To the west of Eight Mile canyon the Silurian rocks reach far up on the mountain slopes, ap-parently because of the protecting sheet of Fre-mont limestone which is here present above theHarding sandstone.
Crossing the lower part of Eight Mile canyon isan overthrust fault with a northeast-southwesttrend and steep dip which passes into a fold ateither end, but has produced a considerable dislo-cation in its central part, bringing up the crystal-line rocks on the southeastern side.
Garden park bay. —Wrapping around the endof the Colorado range the sedimentary formationsextend northward into an old bay the boundariesof which have varied greatly at different times.The principal element in the present structure ofthe sedimentary formations of this bay is a flatsyncline whose axis pitches at a low angle in asoutherly direction. At the mouth of the bayare seen the Colorado and Dakota rocks of theCretaceous period, the latter forming a regular dipslope on either side of Oil creek, bounded by cliffswhich are well expressed upon the map. Hear theborder of the map on both sides of this synclinethe Dakota assumes a steeper dip, forming charac-teristic hogback ridges, one extending southwardto Canyon city, and the other curving eastwardto a connection with the mesa of the southeastcorner. Below the Dakota the Morrison formationextends a long distance into the old bay, withremnants of the Dakota here and there upon it.In this formation near Oil creek are the famousDinosaur beds from which great numbers of typespecimens have been obtained. Oil has beenfound in small quantity in the Morrison beds notfar below the Dakota, on the eastern side of Oilcreek.
The Fountain grits form the floor of Cardenpark proper and extend northward until cut offby a transverse fault. The northern part of theCarden park bay contains three Silurian forma-tions exposed in southerly dipping mesas, cut bycanyons affording excellent exposures. In Bedridge, at the extreme northern limit of the bay, aremnant of Fountain beds rests nearly horizontallyupon the edges of the Silurian formations. Thepreservation of the Fountain beds is due to arhyolite sheet above them.
The present limitations of the sedimentary for-mations of Carden park bay are chiefly determinedby a complicated system of faults. These havelowered the bay area with respect to the granitemountains on either side. They are situatedprincipally at the foot of steep monoclinal folds oneither side of the syncline. Variations in strike ofthe steeper inclined beds on the mountain sideshave brought them against the fault in severalplaces in such a way that the formations havebeen successively sheared off in the manner repre-sented upon the map. The principal faults trendnearly north and south, but several smaller trans-verse faults have greatly complicated the structure.The great fault east of Helena canyon is mostclearly marked. Its throw near Wilson creek
amounts to nearly 1,000 feet, but greatly dimin-ishes southward to the vicinity of Millsap creek,where it passes into a fold. To the north of Wil-son creek within the semicircular curve in thecourse of this fault there has been a slight over-thrust movement of the granite upon the sedi-mentary beds. Profile sections DD and EE of thestructure section sheet cross Garden park bay.
Twelve Mile paid. —In Twelve Mile park and ex-tending over the little divide into West EightMile park is an isolated area of Mesozoic strata ina synclinal basin. The Morrison, Dakota, Colorado and Montana formations are present, but on thesouthwestern side the Colorado shales apparentlyoverlap the lower formations along a shore line.Hear Bumback Springs a small remnant of Mani-tou limestone is found dipping beneath theMorrison beds. All this area was once directlyconnected with the Mesozoic of Garden park bayover the low divide, and also with the strata nowfound about Parkdale in the Arkansas valley tothe south.
Remnants upon the plateau. —Hear the southernend of the Colorado range several small remnantsof Silurian limestone have been found in nearlyhorizontal position, at elevations up to somethingmore than 9,000 feet. The highest and mostnortherly of these remnants is on the ridge east ofLower Beaver park. Another appears one milesoutheast of Hippie mountain, while severalsmaller ones occur on the southern spurs of Coopermountain. The largest patch appears on an un-named mountain east of Eight Mile canyon.
In the last named remnant the Harding sand-stone is present with a very thin layer of Manitoulimestone below it, and in all other cases it isusually the cherty lower layer of the Manitouwhich is preserved. Similar remnants found be-neath the Morrison beds near Bumback Springsand on the south side of Wilson park, show thatthe Silurian ocean of the Manitou and perhaps ofother epochs covered considerable territory oneither side of Garden Park bay. These remnantshave been isolated by erosion in the arch of themonoclinal fold, probably during the interval be-tween the Millsap and Fountain epochs of theCarboniferous period.
West of Oil creek a patch of Dakota sandstoneoccurs on the south side of Wilson park; twopatches of Morrison and Dakota occur on WestWilson creek southeast of Cap rock; in and nearHigh park are several small remnants of the Mor-rison beds; and in The Basin, below the volcanicbreccia, is a well characterized exposure of Dakotaand Colorado (Cretaceous ) strata. All of theseremnants occur either in park depressions or wherethey have been protected by overlying volcanicrocks. They seem to represent deposits in hollowsof a sea whose floor was gradually sinking.
Manitou park basin. —In the northeasterncorner of the district, near the head of Fountaincreek, occurs the southern end of the Manitou parkbasin of Silurian and Carboniferous rocks. Thebasin structure now seen is apparently due entire-ly to folding, and the strata are supposed to havebeen connected with the main areas to the east.On the eastern edge of this basin the Manitoulimestone appears as a narrow band in verticalposition. Above it the Fountain grits possess agreatly diminished dip toward the west and on theeastern line abut against the granite. Whetherthis western contact represents a fault or anancient shore line is not definitely known. Thebasin ends to the south on account of the north-erly pitch of the synclinal axis. The structure isshown in section AA of the structure sectionsheet.
Eocene and Neocene lake basins. —The largest ofthe post-Cretaceous lake basins, that about Florris-sant, has been quite fully described in discussingthe character of the sediments. It does not appear tohave been at any time much larger than is indicatedby the strata now preserved. The Alnwick lakein the valley of Oil creek appears also to havebeen limited by topography very much like thatof the present day. The lake of High park, how-ever, had an extension to the southeast as shownby the remnant of its lowest conglomerate foundupon the rhyolite sheet capping Bed ridge.There is no evidence concerning its former limita-tion in this direction. Apparently this lake musthave had a considerable body of water, for thechief constituent of its conglomerate is the ex-tremely hard Algonkian quartzite which occurs
only in small perfectly rounded pebbles, affordingevidence of a great amount of attrition. There isat present a difference of 500 feet in elevation be-tween the floor of High park and the summit ofBed ridge.
AREAS OF VOLCANIC ROCKS.
Western central area. —The largest area of vol-canic rocks in the district represents the easternextremity of a volcanic formation which occupiesthe greater part of the elevated country betweenSouth park and the Arkansas river. The princi-pal formation in this volcanic complex is a dark,basic breccia and agglomerate with numerous in-tercalated basalt sheets and dikes. This agglomer-ate accumulated on an irregular floor of graniteand gneiss, as proved by the local projection ofgranite through it and the occurrence of the ag-glomerate in valleys and hollows. The basicbreccia forms mountains and mesas, the largestmasses lying, however, west of the area mapped.
Horth of Four Mile creek are several mountainscomposed of the agglomerate with capping sheetsof andesite. To the south the agglomerate formsthe main part of Wicher mountain, and in itsfurther extension produces the large dissectedmesas on either side of West Wilson creek. Thelarge intercalated andesite sheet has been an im-portant element in the preservation of these mesas.
Massive andesite and trachyte also form hillsand mountains within this general area. Theserocks are more recent than the earliest portions ofthe great breccia formation and are distinctlyolder than the upper part of the fragmental de-posits. Dikes of these rocks penetrate the lowerpart of the breccia in great abundance. Bhyoliteoccurs in small masses associated with the othervolcanics, apparently in remnants of an extensivebut thin surface flow. There is a local center ofandesitic eruption on the south side of Four Milecreek opposite Truro. The rocks of this largevolcanic region are as a rule very fresh.
High park. —Hear the center of the High parkdepression are some prominent hills composed of arather fine grained tuff and agglomerate chiefly ofandesitic material. Southward from these hillsthe even floor of the park is covered by a thinrhyolite flow, upon which the High Park lake bedswere deposited. The relative age of the volcanicrocks here exposed was not positively determined.But the absence of andesitic debris in the sedi-ments of the High park lake seems to show thatthe andesitic tuffs are younger than the rhyolite.In the hill on the eastern side of the park the lakebeds are found between flows of rhyolite andphonolite.
The Bare hills. —Southward from High parkis a group of bare, grassy hills upon the site of avent of andesitic eruption. The earliest outburstat this point produced a fine grained, light coloredtuff full of fragments of granite. Penetratingthis tuff are several necks and dikes of densehornblende-mica-andesite. The surface masseserupted through these channels built up the higherportions of the hills resting upon the tuff or uponthe outlying granite.
Southwestern area. —In the extreme southwest-ern corner of the district is a narrow mesa whichis capped by several thin sheets of olivine-bearingandesite which rest upon rhyolitic breccia and tuff,beneath which on the northern side are remnantsof a dark basic breccia seemingly equivalent withthat to the north. On the northern side of Talla-hassee creek rhyolite and basic breccia seem to al-ternate. Both of these isolated patches belong tothe great western volcanic district the margins ofwhich approach very near the border of the map.
Chippie Creek district. —The group of hills con-taining the mines of the Cripple Creek region arechiefly made up of a much decomposed andesitictuff, breccia, and agglomerate formation, withsmall bodies of massive andesite, and penetratedby numerous dikes of phonolite. Detailed de-scriptions of the central portion of this region ac-company the special map of this folio. Beyondthe borders of the special map there are, however,mauy phonolite masses which are distinctlygrouped about this center, and are to be consideredas belonging to it.
The outlying phonolite masses range from apoint on the slopes of Pikes peak itself, sevenmiles northeast of Bull mountain, to Hippie moun-tain on the south and High park on the west.Hearly all of the larger bodies seem to be surfacemasses from sources immediately below them.
East of Little Pisgah peak there is a large irregu-lar neck of phonolite, cutting the granite in therugged cliffs facing Wilson creek. A great manydikes of phonolite are found in granite on all sidesof the volcanic center. Traces of rhyolitic tuffand breccia found at a few points indicate the al-most complete destruction of a formation of thischaracter which may have been quite extensive.
Scattered remnants of rhyolite. —The geologicmap shows a large number of very small patchesof rhyolite, most numerous in the vicinity of theFlorissant lake, but also noticed as far south as theeastern ridges of the Bare hills. The rock of allof these patches indicates a finely banded surfaceflow of no great thickness. The rock is very uni-form in appearance throughout the region, but theremnants found occur at many different elevationsentirely independent of present topography. Inhow far the differences of level observed havebeen produced by faults undetected in the graniteit is impossible to say, but the fact that the rhyolitecapping of Bed ridge bears upon it a conglomer-ate apparently identical with the High park con-glomerate, indicates a dislocation between thesepoints of nearly 500 feet, and it is quite probablethat many other differences in elevation of rhyo-lite remnants are due to the same cause.
STRUCTURAL DEVELOPMENTOF THE REGION.
Ho approximately complete sketch of the geolog-ical history of this region can as yet be written,owing to the reconnoissance character of the ob-servations thus far made, and the obscurity of theevidence concerning some of the most importantgeological periods. The data that have been pre-sented, however, throw new light upon the historyof certain periods, and a summary review of thesuccession of events as thus indicated will begiven.
Age of the granites and gneisses. —The granitesand gneisses of the Bocky mountains have alwaysbeen considered to be a part of the Archean , theoldest known complex of rocks, whether deemedto be plutonic eruptives or of metamorphic origin.But a remarkable feature of the area of the Pikespeak sheet, now observed for the first time, is theoccurrence in the main granitic masses of anenormous number of included fragments ofquartzite and of schists derived from them. Thepurer quartzites are plainly indurated sandstones,necessarily older than the granites which includethem. From considerations presented elsewherethese ancient sediments have been referred to theAlgonkian period. The granites cutting thesestrata are then either of Algonkian age or of earlyCambrian , and it is deemed most in harmony withthe other facts in the case to refer the granite erup-tions to the later Algonkian period. With suchan age assigned to the granites the dynamic move-ment which produced gneisses out of them maywell be considered indentical with that which pre-ceded the Cambrian . It is known that the gneissicstructure in question was produced before theperiod of the upper Cambrian , during which timesediments were deposited upon these gneisses inthe Colorado range.
Bre-Paleozoic land areas. —The fragments ofquartzite, schist and gneiss included in the granitemust be regarded as representing the oldest knownrocks of this portion of the Colorado range. Butit is impossible from present knowledge even ap-proximately to define the continental areas of theperiod of sedimentation, or the extent of the de-posits. The enormous granite masses which brokeinto and through these oldest formations doubt-less formed extensive land areas, but it seemsprobable that the great dynamic movements whichtransformed so large a part of these massivegranites into banded gneisses were the ones whichgave outline to the continents about which theupper Cambrian and Silurian strata were de-posited. Much further investigation is necessaryto determine how large a part of the gneisses inother portions of the Bocky mountains has beenformed from granites contemporary in origin withthose of the Pikes peak district.
The mam structural axis of this portion of theBocky. mountains has a trend north-northwestand it is parallel to this general direction that thebanded structure of the gneisses has been de-veloped. There are many minor deviations fromthis rule, but the broad correspondence in struct-ures is nevertheless plain. From the upper