upper side of many of them and drainage ravineswinding intricately around among them. In thefrontispiece to Mr. Purington’s paper in the Eigh-teenth Annual Report this topography is illus-trated, as seen from San Bernardo Mountain.Below the 10,400-foot level large blocks of SanMiguel conglomerate are numerous, and surfacesof considerable extent show only debris of thisformation, but perhaps still lower may be foundan area equally characterized by the San Juantuff. The larger knolls may have ledge-like out-crops, showing that either the San Juan or theSan Miguel formation is present in mass, but thedips vary, and in the strike of a ledge of one ofthese are knolls exhibiting the other rocks. Nooutcrops of shale could be found down to thelevel of Trout Lake.
On the ridge running west from Yellow Moun-tain a small block of the San Miguel has droppeda distance somewhat less than the thick- .
Adjacent
ness of the formation. It is much fis- slide b,ocks -sured and will surely be precipitated down thesteep slope at no distant day. A few hundredfeet below that a similar block of the San Miguelrests on the Mancos shales, marking an earlierslide, and in the valley of Lake Fork is an area,shown by the map, within which is a jumble oflarge and small fragments of the San Miguel con-glomerate, thought to represent a slide block inan advanced state of disintegration.
The lower border of the large slide area east ofTrout Lake is indicated on the map by a lineshowing the lower limit of a confused mass ofSan Juan and San Miguel materials which con-tinuously conceal the Mancos shales. From thenature of the case this line can be drawn withonly approximate exactness. Spots of debrisoccur in places below it, and the grassy or woodedslopes above it may in some places be underlainby shale. It is supposed that the shales beneaththe San Miguel in Ground Hog Gulch belong tothe landslide mass, on account of their dip.
The southern boundary of this mass as repre-sented on the map is possibly incorrect. The linemay be in Leslie Gulch instead of in PovertyGulch, as drawn. Observations of sufficientdetail to determine this point were not made.
Reviewing the evidence of this area, it seemsthat a great slide has taken place, and either atthe time it occurred or subsequently the blockhas been much broken up, so that only the gen-eral relations of the various parts can now bemade out. The base of the rhyolite series inGround Hog Gulch is 1000 feet vertically belowits normal place on the ridge above, but the baseof the San Miguel is only 200 or 300 feet lowerthan the level it occupies to the north or south ofthe landslide block. This discrepancy is partlyaccounted for by the increasing dip of the lowerpart of the dislocated complex, but a larger factorin this case is probably the grinding up and dis-appearance from the section of much of the SanJuan tuff, a result of the complex fracturingwhich the slide block as a whole has undergone.
Landslide area northwest of Silver Mountain .—The largest landslide area of the Telluride quad-rangle, embracing about 10 square miles andshown on the map, extends from Lake Forknear Ames nearly to the Telluride branch of theSan Miguel. Its upper limit is along the slopesof Silver Mountain, Bald Mountain, and GoldHill, and its lower border is in general about1500 feet below, except at the southern end,where the slide material comes quite down to thestream of Lake Fork, more than 2500 feet belowthe upper line as it crosses Turkey Creek.
The topography within this area is that mostnaturally characteristic of a surface made up oflandslide blocks. In fig. 10 is illustrated the con-figuration of the southern part of the landslidemass as seen from the western side of Lake Fork,looking toward Silver Mountain. There are agreat number of knolls, longitudinal ridges, orbenches, the majority of which have steep outerslopes, with trenches, or depressions, often contain-ing a stagnant pool, back of them, on the moun-tain side, and the drainage is extremely irregular.
While this area is large it exhibits relativelyfew exposures in which the attitude of the beddedformations there present can be clearlv
d Observations
seen. On the upper limit, north of inthisafea -Ophir Needles, a small slide block of San Juantuff, etc., interrupts the San Miguel ledge betweenthe levels 10,500 and 11,000 feet. In this blockthe tuffs strike somewhat west of north and dip
21° easterly. This seems to have been a recentslide of the lower few hundred feet of the SanJuan formation. The trail from the Gold Kingroad to Ophir Loop passes at the base of thisslide block, over a bench on which there is a smalllake. At about this level to the north is a muchmore extensive bench with two ponds, which areshown upon the map. These benches are typicalof many in this slide area, having a steep outerslope in which the crushed San Juan tuff does notform a distinct ledge outcrop, though its presenceis plain.
From the south branch of Turkey Creek aroundBald Mountain to Prospect Creek the slide line isnot sharply indicated, except that above it areseen nearly continuous outcrops of the San Juantuff in normal position and below it the confusedlandslide topography begins. On the west slopeof Gold Hill, however, lateral ridges, with a trenchback of them, are found in several places near thecliffs of the San Juan. In some of these ridgesthe outcrops show the San Juan tuff dipping atvarious decided angles toward Gold Hill. One ofthese is below the point where the normal cliff ofthe San Miguel formation reappears.
Rhyolitic debris is scattered over the upperpart of the slide area, but was not found in mass,as east of Trout Lake. San Juan tuff and agglom-erate forms most of the knolls and benches downto a level somewhat below 10,500 feet. In severaloutcrops below the Gold King road dips of asmuch as 60°, generally somewhat north of east,were observed.
The most regular element in the composition ofthis area seems to be the presence of the SanMiguel formation in a broad band extendingfrom Prospect Creek to Turkey Creek. The lowerline of the slide and the line between the SanMiguel and San Juan formations can be prettyclearly made out within this space. Mr. Spencernoted several outcrops of the San Miguel havingstrikes between N. 15° W. and N. 20° W., withdips of 45° to 60° easterly.
South of the south branch of Turkey Creek noregular relation between San Juan and San Migueloutcrops can be made out. Above the Currencymine is a knoll of San Miguel conglomerate withirregular eastward dip, the reddish western facebeing visible miles away. This is the ovalmass represented upon the map. All around it isdebris of the San Juan tuff.
The Currency mine (No. 30 on Economic sheet),a little below this exposure, has a shaft 200 feetdeep sunk through San Juan and San The CurrencyMiguel formations to the Mancos shales. mine -By an unfortunate error on the Economic sheetthe Currency shaft has been placed north of theboundary of the landslide. It belongs withinthe landslide area, immediately below the knoll ofthe San Miguel conglomerate. This shaft wassunk on an ore-bearing vein which was traceddown to the Mancos shale, though much fracturedand dislocated. Drifts from the shaft and varioussurface workings have shown the broken-up char-acter of the San Juan formation about this pointand have furnished evidence of slipping still inprogress. Thus tunnels running nearly parallelto the general slope of the country in the vicinityexhibit a crushing of the timbers, especially onthe upper or mountain side. The Currency shaftcould not be kept vertical, the bottom movingdown the slope and with a twist indicating someundulation in the shale surface upon which thesliding mass rests. A short distance from theCurrency shaft a prospect tunnel in broken-upSan Juan material was run into an older tunnelwhose timbers were all crushed together, theentrance to this old working having been entirelyobliterated by recent sliding. Mancos shale wasalso found here, much nearer the surface than inthe Currency shaft.
The attempts to find a continuous ore body inthe Currency mine resulted in the discovery ofsufficient ore in small stringers and dis- XheCurrencyconnected masses to induce the erection ore body -of a steam hoisting plant and other mine machin-ery, but the total irregularity in dislocation andthe cumulative evidence that the whole mass wasslide material caused the abandonment of furtherexploration. There are some indications ofsecondary deposition of ore on the shale contact,but the thoroughly crushed condition of vein androck matter and the likelihood of some ore beingdragged into the zones of movement make posi-tive statements on this point unwarrantable.
Material representing the San Miguel conglom-erate, much broken up, occupied some 15 feetabove the shale in the Currency shaft. In somelevels and tunnels a greater thickness was found,but in both this and the San Juan tuffs, etc., thenormal bedded structure could seldom be madeout, and the innumerable fracture planes runningin all directions plainly showed the cause of theexisting conditions. It seems probable that theCurrency mine is in a slide block which may haveoriginally been much more solid than at present,and that secondary sliding has increased the dis-location upon the fractures produced in the fall,and perhaps created new ones.
Below the Currency mine the confused min-gling of San Juan, San Miguel, and Mancos shaleextends down to Lake Fork. At one point thewagon road close by the stream passes over theloose black shale 500 feet below the Dakotaledge, over which it has been pushed by the slid-ing mass of the other formations. This shale andthat of other exposures higher on the slope isthought to belong to the great furrow of this softmaterial which must have been thrown up by theplowing force of the original slide.
There is much landslide material in disconnectedmasses on the slope west of Ophir Needles, andthe southern border for the landslide mass, asshown on the map, is thus a necessary general-ization.
From the extent and observed structure of this,the largest landslide area of the Telluride quad-rangle, it is supposed to be the product .
° 7 L L A compound
of several slides from the mountain face, ,ands,ide -which were perhaps contemporaneous or nearly so,and that ever since the primary slide there hasbeen continual slipping of minor masses withinthe area. The disintegration of the whole massis going on, under the active operation of a geo-logical agent of no mean importance in such aregion.
Landslide at west base of Sheep Mountain .—The map shows a small landslide area just belowthe cliff of the San Miguel formation Rockgon the west slope of Sheep Mountain. exposed ’
In this mass Potosi rhyolite flows and tuffs, theSan Juan tuff, etc., and some coarser agglomeratereferred to the Intermediate series, have been dis-tinguished and represented upon the map in theirproper colors. The main part of this mass isPotosi rhyolite, of two lamellar flows, with theusual flow-breccia below them. Under this rhyo-lite comes some very much crushed San Juan tuffin a thin band less than 200 feet thick at itsmaximum. Bowlders and pebbles of quartzitesand granite found at its base in a few spots indi-cate a disintegrated layer of the San Miguel con-glomerate. Across the southern end of the rhyo-lite flows runs a cross fracture, beyond which achaotic agglomerate of pyroxene-andesite appears,seeming to belong to the Intermediate member ofthe volcanic complex. The lower boundary ofthis mass is obscure, in a heavy growth of spruceforest, but shale exposures at several points andthe smooth character of the slopes below indicatethat the map is here nearly correct.
The laminated rhyolite flows dip toward SheepMountain, but not regularly, since cross fracturesplainly show the mass to be much broken up. Onthe west face of Sheep Mountain above this slideis a depression drained by two shallow ravines,shown on the map, and one can not avoid theconclusion that the depression marks the placefrom which the slide block came. If the originalmass included much of - the San Juan, that mate-rial was so ground up by friction that little of itnow remains.
This block shows that at the time of its fallthe Potosi series extended out along the crestfrom San Miguel Peak to Sheep Mountain. It isnot now present at the extremity of Sheep Moun-tain, and only small remnants may be still foundin the points above 13,500 feet near San MiguelPeak. Much less erosion has been necessary toremove this rock from Sheep Mountain than musthave taken place on the Yellow Mountain-PilotKnob ridge.
Other landslide masses. — Other landslidesbesides those above described have undoubtedlytaken place at many localities along the San Juanfront, but they are either so small or have becomeso disintegrated in course of time that they cannot be represented on the map. One point wheresmall masses have been detached and have brokeninto smaller blocks in their fall is on the unnamed
mountain northwest of Grizzly Peak. Here areseveral pinnacled projections in the zone of theSan Miguel formation, already detached by cre-vasses from the mountain and slightly dislocated,which must ere long fall or slide en masse downthe steep shale slope beneath.
It is quite probable that small slides, more orless broken up, exist on the north slope of SheepMountain and to the north of the San MiguelRiver, on the rounded slope below the cliffs seenin fig. 1.
THE MOUNT WILSON GROUP AND ADJACENTSUMMITS.
It has been stated above that the San Miguelformation and some of the overlying volcanicrocks have been found in the Mount Wilson groupand adjacent summits. The geological mapexpresses the observed distribution oftormations, demonstrating at once that thesan j uanthe Mount Wilson group is geologicallyan outlier of the San Juan Mountains , isolated bythe deep erosion of the San Miguel and Doloresrivers. A glance at the map will show the almostcomplete correspondence in constitution betweenthe mountains under discussion and the south-western promontory of the San Juan ending inGrizzly Peak. In each area a great stock of gran-ular rock is partly surrounded by remnants of theSan Miguel formation and of the overlying bed-ded volcanics. In the one case connection withthe main mass of the San Juan still exists; in theother the isolation is complete.
Summits west of Trout Lake. —To the west ofTrout Lake is a long, curving ridge with an abruptface on the southern and southeastern sides andwith great talus streams extending far out overthe lower shale slopes. This ridge is due to agreat irregular injection of gabbro-diorite in Man-cos shale. Just under its summit on the southside a dense, dark igneous rock allied to campton-ite penetrates the gabbro-diorite in an irregularsmall stock or plug. Its vertical cliffs contraststrongly with the gray mass of the
mountain and form so prominent a fea-ture as seen from the south that the hithertonameless elevation- has been called Black Face .This mass of gabbro-diorite is plainly to be con-sidered as an arm of the Yellow Mountain stock,but the contacts are seldom visible. At the northend, above San Bernardo, the shales are seen in aravine, and are much contorted and baked nearthe intrusive mass. The shales may also be seenabove the mass, dipping northwesterly from thesummit of Black Face and easterly from the hillsouth of the Lizard Head. On the southeast sideof the latter hill the lower contact is also seen,and on the northwest a thin branch in sheet formis present in the shales. At the end toward theLizard Head the mass comes in contact with theSan Miguel conglomerate and sends off a narrowdike into it. At several places in this dark-graygabbro-diorite are coarse-grained veins or smallsegregations rich in orthoclase and quartz, oftenintergrown in the form of graphic granite.
The first trace of the San Miguel conglomeratewest of Lake Fork is in San Bernardo Mountain,which is capped by a remnant of thatformation some 400 feet in thickness, and Sunshine
. , . , . mountains.
its base being about 500 ieet higherthan on the west slope of Yellow Mountain.West of San Bernardo Mountain Wilson Creek hascut down 1200 feet into the soft Mancos shales.On its western side rises a high ridge, at thenorthern end of which is Sunshine Mountain andat the southern end the singular monolith calledthe Lizard Head. In this ridge the San Miguelformation has a variable development. Under thesummit of Sunshine Mountain the formation hasa thickness of nearly 1000 feet, while at thesouthern base of the Lizard Head the San Juantuffs come down to less than 200 feet of the Man-cos shales. This seems explainable only as dueto erosion, and indicates that the San Juan tuffswere not deposited at this point in such perfectcontinuity with the San Miguel beds as theyappear to have been elsewhere.
A thin remnant of rhyolitic flow-breccia andmixed tuffs of the Intermediate series occurs onthe ridge about the Lizard Head. The The Lizardlatter is a column with nearly vertical Head *walls on all sides, rising nearly 300 feet aboveits platform. Its summit is inaccessible, and thereason for its preservation is not evident. At thebase it is a bedded mass of andesitic breccia,which might belong to the Intermediate series,
Telluride—11.