296 Keyes — Significance of Certain Unconformities. 



Art. XXV. — Orotaxial Significance of Certain Unconform- 

 ities • by Charles R. Keyes. 



The unconformable relationships which some of the greater 

 geological formations bear to one another in the Rocky Moun- 

 tain region have been widely noted. Most of the observations 

 bearing upon this theme have been made in Colorado and the 

 states to the north and west. In this region, however, the full 

 significance of the various unconformities which have been rec- 

 ognized cannot be measured. In each instance the values are 

 qualitative and not quantitative. 



The region mentioned has continued to be the theater of 

 mountain-building from very early geological times. For this 

 reason very largely the record of its geologic history is broken 

 and obscured, and it is necessary to go into neighboring states 

 to complete the account. To the southward in New Mexico 

 conditions are quite different. 



Soon after passing the south Colorado boundary the south- 

 ern Rocky Mountains rapidly lose their imposing character and 

 the last vestige is seen in the pitching anticline near the Glori- 

 etta pass, on the Atchison, Topeka and Santa Fe railroad, a 

 few miles from the city of Santa Fe. Southward from this 

 point, even to beyond the Mexican boundary 300 miles away, 

 deposition has been more complete and continuous and 

 repeated erosion less destructive than further northward. 



In a given region it has been customary to write its geologi- 

 cal history from the sediments alone. Gaps, even when recog- 

 nized, have been passed over with little or no comment. 

 Geologic history has, however, an erosional record that is 

 about as long and fully important as the depositional record. 

 The preserved erosional surfaces are the planes of unconformity. 

 It is to some of the most important of these phenomena, as 

 they have been recently made out beyond the southern Rocky 

 Mountains, and to some of their depositional equivalents that 

 attention is here called. 



A decade and a half ago, S. F. Emmons, in a paper read 

 before the Geological Society of America* on the Orographic 

 Movements in the Rocky Mountains, mentioned the fact that 

 from earliest Archean times this region has been one of 

 upheaval. The author above mentioned, after reviewing the 

 geological literature relating to this broad expanse of country, 

 notes in the general geological column ten more or less marked 

 unconformities. 



In recent years, the same subject has been again broached, 

 but from a somewhat different standpoint and in a neighboring 

 province. More than double Emmons' number of great uncon- 



*Bull. Geol. Soc. America, vol. i, pp. 245-286, 1890. 



