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Show 164 EXPLORATION OF THE OARONS OF THE COLORADO. A part of the district in which my observat.ions were made has since beer; much more thoroughly studied by Mr. Archibald R. Marvine, one of tho geologists of the First Division of tho "Geological and Ooograpbical Survey of the Territories." In· his report of June lV, 1874, ho says: "Three causes combine to render the rapid study of tho stratigraphy of the archroan rocks difficult and its results uncertain: Fir t, their structure is not only often complex, but obscure, the evidence of it being at times nearly or wholly obliterated by the metamorphism, and often over largo areas very difficult to find; second, thi metamorphism renders lithological characters inconstant, so that a stratum that at one point may be characteristic among its neighbors, may, at another, become like them, or all may change so as to reta.in none of their geological features, becoming again like other series, so that lithological resemblances cannot often be taken as a guide to follow, and may even becomo misleading; third, tho erosion producing the present smface features of the mountain region lntd the direction of its action determined by movements of the surface which wero not closely connected with tho extended plications of it rocks ; and moreover since this erosion has not long been acting among these rocks, ;l1~re appear~ no well defined connection between the topography and the structural geology. The ancient erosion gradually wore down the mass to tho surface of the sea, and while previously to thi~ it was no doubt directed by the structure, yet the mass was finally leveled off irrespective of structure or relative hardness of its beds by the encroachincr oce"n 1 · 1 1 d o ... , w uc 1 wo1·co over its .rui~s and laid them down upon the smoothed surface in tho form of the Tnass1~ a~d other beds. The recent great uplift, while it probably added new pltcatwns to the accumulated plications of tho t • tl · . . . pas .. m 10 anment rocks, :v~s q.mte stmple With respect to their total plication, and loft the upper TI?aSSIC and other sedimentary beds COmparatively simply structured, they havmg been affected alone by the later movements "As the mass appeared above the sea and surface. erosion on~ more commenced, bnt which now acts upon the t k . . recen roc s coverrncr probabl 11~ gref,ater part the complex underlying rocks, it was directed o~ from tl~ me o greater uplift down the 1 · · ' ong s1 o pes of tho rising continent to tho retm.ng sea. The channels of drainage started wero directed solely by the I MARVINE ON SUPERIMPOSED VALLEYS. 165 structure and characters of the upper rocks, and w hr.n thoy gradually cut down throno·h these and commenced sinking their canon into tho under-o l . lying complicated rocks, these canons boro no relation whatever to t 101r complications. It is but recently that the upper rocks havo been completely removed from tho summits of the mountain-spurs, tho ancient level of subaqueous. erosion being still indicated by the often uniform. level o.f the sp~t rs and hill-tops over considerable areas, and large platcau-ltko r g10ns wh1ch be.came very marked from Certain points of view. rrwo Or t.hr e U~h )OYels are indicated at a fow places, showing not only that tho s dtmontanes have once extended up over what aro now tho mountain rocks, but that tl~o uplifting has been mainly confined along certain p~r·~ly woll-defin.ed lme~, the intermediate bolts, though uplifted bodily, remmn111g comparattvely level, a typo of folding, probably, not uncommon farther wost, and which will bo referred to again in tho following chapter. * * * * * * * "It is true that the structure of the lower rocks has begun to affect the courses o f tho Streams, 'a nd in places to a considerable extent. Me. eting .a softer bed a canon will often have its course directed by it, and follow 1t £o r some d1. st ance , ]eav1·nc0 r the adJ·acont harder beds plainly indica•t ed by t• ho :d nd sometimes the inuosities of structure are very cm·tously fo l- II ges, a l lowed by a stream in all its windings, but it soon bre~ks away an< m~~ · d d tl f tl 0 beddincr Many of tho smaller ntNmes have had tlwn m open en y o 1 o· . d .. · 0 • • post· tl· ons d e t erm·m e d by the structure·, but m a broad sonso the t,t11Ut,0 e rs f· m t.ho main mountain crest eastward, independent of sb·ucturo. 'I hus, 10 while in place geological features may find expressi·O n m· sur f:a .co f orm, y et, as often, there may bo no conceivable relation between topography and . 1 cr rrho subaqueous erosion, in smoothing a ll to a common lovel, goo OoY· . d I destroys all former ~urfu.ce expression of goolo~1cal cllat·acte:·, ~t~ t 10 present eros·to n 1m s no t y et been in procor ress suffic10ntly long to IOCtoate the lo~: features.'' . I fully concur with Mr. Marvine in the abovo oxplanatron of. the ~alleys m· the mam· R oc1 c y M oun t at· ns of Colorado, as my own observatiOn!:i m that country 11 a d 1e d m 0 to t lle Same conclusioll. rn wre can b.o. no donbt that tho present COUI.S OS 0 f the streams were determined by coud1tlons not fountl ' |
