114 PROCEEDINGS OF THE PHILADELPHIA MEETING 



There are two fiiiidanientally different series of formations separated by an 

 uiiconforniity. The older series is a complex of tuffs, shales, conglomerates, 

 ;ni(l limestones, cut in a complex way by very numerous and occasionally very 

 large intrusive masses, chiefly of dioritic composition. In many parts of the 

 Lslaud these beds show complicated structural relations and much disturbance. 

 The younger series is of Tertiary age and is essentially a succession of marls 

 and limestone reefs and shale beds of considerable variety, but not affected 

 at all by igneous activity or very complicated dynamic disturbances. 



Peculiar erosional effects are produced in certain districts where this latter 

 formation is the imderlying rock, and in some districts the inner margin of 

 the limestone belt, which is usually a narrow strip along the coast, develops 

 into a pronounced cuesta form. The unconformity between the two great 

 series is represented by an obscurely marked peneplain, which bevels across 

 all of the complicated structures of the older series ; but, except in the im- 

 mediate vicinity of the limestone margin, there is scarcely a trace of this plain 

 to be seen, because of the maturity of dissection. 



A large number of photographs were taken to represent the characteristic 

 features, and a collection of typical rocks and fossils was made which are now 

 l)eing studied. 



Presented in abstract extempnraneousl3\ 



Discussed by Messrs. Cbarles Schucbert and Gilbert van Ingen. 



RELATION OF CRETACEOUS FORMATIONS TO THE ROCKY MOUNTAINS IN 



COLORADO AND NEW MEXICO 



BY WILLIS T. LEE 



(Abstract) 



The paper presented evidence that the unconformity in the Rocky Mountain 

 region, known as the post-La ramie. post-Vermejo, or post-Cretaceous, according 

 to locality, is of such magnitude and extent as appropriately to constitute the 

 Cretaceous-Tertiary boundary for this region. In opposition to the view that 

 highlands or large "islands" persisted in the Rocky Mountain region through- 

 out Cretaceous time, the facts now available indicate that the Dakota sand- 

 stone was laid down on a baseleveled surface, and that it originally extended 

 continuously over the present mountainous area of Colorado and New Mexico ; 

 also that the succeeding beds of Upper Cretaceous age covered this area. 

 Whatever surface warpings may have occurred during Cretaceous time, there 

 seems to have been formed in the Rocky Mountain region no barrier that 

 seriously interfered with the free distribution of the sediments in the interior 

 Cretaceous sea. When the post-Laramie uplift occurred these sediments must 

 have been removed before the pre-Cretaceous rocks could furnish the pebbles 

 found in the basal conglomerate of the post-Laramie formations. If the Cre- 

 taceous sedimentaries extended over the mountains with anything like the 

 thicknesses found on either side and also within the mountains, the uplift 

 necessai'y to obtain these pebbles indicates an orogenic movement of such 

 magnitude as to denote the close of the long period of Cretaceous quiescence 

 and to inaugurate the tumultuous period of orogenic disturbances of the 

 Tertiary. 



Presented in abstract extemporaneously. 



