UNCONFORMITY OF CRETACEOUS AND TERTIARY. 1 1 



fossils of purely fresh- water types The junction of the two series is xincon- 

 formable, and is often highly so. This unconformity is seen in many 

 localities on both sides of the Uintas, along the eastern slopes of the 

 Wasatch, and becomes even more strongly pronounced to the southwest- 

 ward. During the covirse of this work, localities will be mentioned where 

 it is conspicuously displayed, the Upper Cretaceous (Laramie) beds being 

 flexed at a high angle, the flexures planed off by erosion, and the overlying 

 series resting across the beveled edges, or even upon the Jurassic beds 

 below. It was at this iinconformity that Professor Powell drew the divid- 

 ing horizon between the Tertiary and Cretaceous. Quite independently of 

 any physical break. Professor Meek had chosen the division at the same 

 horizon upon the evidence of invei'tebrate fossils, though that evidence was 

 regarded by him as being too meager and the species too few and indecisive 

 to justify an unqualified opinion.* Professor Marsh also reached a similar 

 conclusion much more decisively from mammalian fossils from beds just 

 above the unconformity which he referred approximately to the horizon of 

 the London clay or the base of the Eocene, t The physical break wliich 

 separates t"liese divisions of time is of wider distribution and more emphatic 

 than was supposed when first detected, for the Upper Cretaceous (^Lara- 

 mie) beds are often greatly flexed and eroded beneath the Tertiary, and 

 these occurrences are frequent throughout the province. Very often, and 

 probably in most of the exposures distant from the mountains, the contact is 

 apparently conformable, for the obvious reason that neither series has been 

 sensibly disturbed from original horizontality, or the disturbances have 

 been of late occurrence, involving both series alike. The separation in 

 such cases then becomes a purely lithological one, or sometimes none can 

 be detected. The fossils do not indicate any break, since the base of the 

 Tertiary and the summit of the Cretaceous are lignitic, and furnish only 

 brackish-water mollusca, which are indecisive and have a very great vert- 

 ical range in nearly all the species. 



* Invertebrate Palieontology (1876), Dr. F. V. Haytley'a Survey, pp. xlvii et seq. 

 tExpl. 4mii Parallel, C. King, vol. ii, p. 329. 



