10 



GUIDEBOOK OF THE WESTERN UNITED STATES. 



indication of the real structure of the mountain range. The streams 

 have cut deep canyons, and many interesting views may be seen on 

 the right of the train as it passes from branch to branch of South 

 Boulder Creek, here crossing a canyon on a high trestle and there 

 plunging into the darkness of a tunnel through a spur. Where 

 South Boulder Creek is first seen it lies far below the level of the 

 road, but its bed slopes steeply headward and is finally crossed by 

 the railroad well above the sharp canyon, which represents the latest 

 period of stream cutting in this region. If the trip is made in July 

 the traveler may have the pleasure of seeing in the foothills acres 

 of the beautiful Rocky Mountain columbine (PI. IV, 5), which has 

 been adopted as the floral emblem of Colorado. The plant grows 

 about 3 feet high, and each stalk bears a number of delicate lavender- 

 tinted blossoms which become white as the season advances. 



The first large village above the point where the railroad crosses 

 South Boulder Creek is RoUinsville. Here the traveler sees no sug- 

 gestions of mining, but if he could follow for a distance of 4 miles the 

 road that climbs the hill on the north (right) he would find himself 

 in a district that furnishes the metal for the filaments of most of 

 the incandescent electric bulbs made in this country. This metal 

 is tungsten, and a small percentage of it is contained in the steel 

 from which most of the modern machine tools are made. 



only in structure and origin. * * * 

 Stinicturally the gneisses are holo- 

 crystalline [entirely crystalline] granu- 

 lar rocks, as are the granites, but dif- 

 fer in that the various constituents 

 are arranged in approximately parallel 

 bands or layers. * * * 



" In width and texture these bands 

 vary indefinitely. It is common to 

 find bands of coarsely crystalline 

 quartz several inches in width, alter- 

 nating with others of feldspar, or feld- 

 spar, quartz, and mica, or hornblende. 

 A lenticular structure Is common, pro- 

 duced by lens-shaped aggregates of 

 quartz or feldspar, about and around 

 which are bent the hornblende or 

 mica laminae [layers]. The rocks 

 vary from finely and evenly fissile 

 through all grades of coarseness and 

 become at times so massive as to be 

 indistinguishable in the hand speci- 

 mens from granites. * * * 



" The origin of gneisses * * * is 

 in many cases somewhat obscure, the 

 banded or foliated structure being con- 

 sidered by some as representing the 



original bedding of the sediments, the 

 different bands representing layers of 

 varying composition. This structui'e 

 is now, however, considered to be due 

 to mechanical causes and in no way 

 dependent upon original stratification. 

 The name, as commonly used, is made 

 to include rocks of widely different 

 structure, which are beyond doubt in 

 part sedimentary and in part eruptive 

 but in all cases altered from their 

 original conditions. 



" This alteration * * * has been 

 brought about not by heat and crystal- 

 lization alone, but in many cases by 

 processes of squeezing, crumpling, and 

 folding so complex as almost to war- 

 rant the application of the term knead- 

 ing. * * * 



" In the present state of our knowl- 

 edge it is in most cases impossible to 

 separate what may be true metamor- 

 phosed sedimentary gneisses from those 

 in which the foliated or banded struc- 

 ture is in no way connected with bed- 

 ding and whicli may or may not be 

 altered eniptives." 



