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ROLLIN T. CHAM BERLIN 



embrace extensive areas of pre-Cambrian granite and other crys- 

 talline rocks. Over such areas of old crystalline rocks it is difficult 

 to reconstruct the folds, since these areas afford only the most 

 meager information as to the nature of the post-Cretaceous folding 

 and as a result are little better than blank pages in the later dias- 

 trophic story. It was therefore advisable to choose a section 

 which, though it be typical of the mountain system as a whole, 



Fig. 4. — Map of the Lyons-Grand River section. The section extends along a 

 curved line from the Colorado and Southern Railroad east of Lyons, to the western 

 slope of the Grand Hogback southwest of Glen wood Springs, a total distance of 135 

 miles. Because of the adopted curvature the Hne of the section crosses the axial lines 

 of the folds approximately at right angles. 



Still crossed the granite belts where they are narrowest, and where 

 the belts of folded sedimentary rocks are widest. This was accom- 

 plished in the section chosen. 



The section finally selected for measurement has as its eastern 

 terminus the bend of the Colorado and Southern Railroad tracks, 

 4^ miles north of the town of Longmont (Niwot sheet, T. 3 N., 

 R. 69 W., Sec. II, middle of S. line). From this point westward 

 the line of section was drawn so as to strike the bench mark (5,375 

 feet) in the town of Lyons, to pass onward through Allen's Park, 

 and across the continental divide one-fifth of a mile north of 

 Oglala Peak (13,147 feet). On the Pacific slope it continues west- 



