BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 10, pp. 141-164, PLS. 14-18 MARCH 23, 1899 



ARCHEAN-CAM BRIAN CONTACT NEAR MANITOU, COLORADO 



BY W. O. CROSBY 



(Read before the Society December S8, 1898) 



CONTENTS 



Page 



Introduction 141 



General structure of the Manitou enibayment 143 



Arcliean-Cainbrian contact 144 



structural details of tlie contact . 145 



Summary of the structural features of the contact 155 



Comparison with other regions 156 



Subaerial versus marine erosion. . 159 



Relation of the form of unconformable contacts to the character of the imme- 

 diately overlvino; sediments 161 



Introduction 



Wliile stud^nng the sandstone dikes accompanying the great fault of 

 Ute pass in the summer of 1896, my attention was naturally attracted 

 to the contact between the granite and the overlying sedimentary rocks, 

 which, as every geological visitor to the region must know, is admirably 

 exposed in the district immediately north of Manitou, on both sides of 

 Ute pass. I was deeply impressed, as would be any student of New 

 England geology, by the wonderful clearness and continuity of the ex- 

 posures, but more especially by the almost absolutely plane form of the 

 contact thus disclosed, and having since enjoyed an opportunity to make 

 a more particular examination of this contact, I am more than ever 

 convinced that it represents a type of erosion unconformity which, al- 

 though probably of frequent and widespread occurrence in nature, is 

 somewhat unique in geological literature, or at least has not received 

 hitherto the attention which it merits. 



The only particular reference to the granite-sedimentary contact of 

 the Manitou district which I have noticed is by Hayden.* In describ- 



*Ann. Rep. U. S. Geol. and Geog. Surv. of the Territories, 1873, p. 35. 

 XXII— Bull. Geoi.. Soc. Am., Vol. 10, 1898 (141 ) 



