18S4.J ^^^ [Stevenson. 



3. Silicious limestone, which can hardly be distinguished 



from quartzite by mere optical examination. 



4. Concealed, 25 feet. 



5. Gneiss. 



The total of the section to the gneiss is about 140 feet. The same section 

 is exposed by a fault at not more than a mile further down the stream, but 

 no traces of change appear. 



Effect of Pressure during Plication. 



The stratigraphical disturbance in the vicinity of Rock creek, a stream 

 flowing amid the Elk range of central Colorado, is extraordinary. The 

 structure was worked out by Mr. Holmes of the U. S. Geological Survey, 

 whose discussion of the matter is one of the best relating to complicated 

 stratigraphy. 



In 1873, the writer made a section across this strange fault-fold. 



Beginning at the mountain top on the east side of the valley and de- 

 scending towards the stream, fifteen strata of sandstone and limestone 

 were found, all wholly unchanged ; the sixteenth in the series is a slightly 

 altered sandstone, while the seventeenth and eighteenth are thick sand- 

 stones, for the most part unchanged, but embracing thin layers of quartzite. 

 Below these are the Cretaceous shales, in the middle of the fold, along the 

 synclinal, but wholly unchanged. 



Beyond the stream the rocks are all changed to some degree and the 

 succession is : 



1. Quartzite, structureless. 



2. Quartzite, imperfect. 



3. Silicious limestone, unchanged. 



4. Quartzite. 



5. Concealed. 



6. Quartzite. 



7. Shale unchanged. 



8. Gypsum, anhydrite. 



9. Limestone, shaly, unchanged. 



10. Quartzite. 



11. Gypsum, anhydrite. 



12. Sandstone, somewhat changed. 



13. Sandstone, much more changed than the last. 



14. Gypsum, anhydrite. 



15. Quartzite, showing no trace of lamination. 



16. White quartzite. 



17. Quartzites alternating with thin limestones; former whollj'' 



changed ; the latter almost unaffected. 



18. Limestone, wholly unchanged 



19. Quartzite wholly changed. 



There are no great dikes here, nor are the beds very near the Archaean 



