32 TF. Cross — Post-Laramie Deposits of Colorado. 



slow erosion of 14,000 feet of strata, from the Laramie clown 

 to the " Red beds " of the Trias.* 



Succeeding the first period of lake-beds came a time of 

 great volcanic outbursts over a very large area. The length 

 of geologic time occupied may not have been very great, but 

 the extent of country in which eruptions occurred at this time, 

 and the great variety of lavas found in the Denver and Middle 

 Park beds, argue for the decided importance of the event as a 

 dynamic manifestation. The position of the Middle Park 

 beds on the edges of the Cretaceous section below the Ft. 

 Pierre, proves a period of erosion there which was no doubt 

 contemporary with the Arapahoe epoch though its record is 

 not known in deposits. Marvine assigns a thickness of over 

 6000 feet to the Middle Park beds (including the " breccia "), 

 characterized through the lower half at least by volcanic ma- 

 terial, — striking testimony to the extent of the eruptions and 

 the duration of the period of deposition, which must have 

 been one of subsidence, approximately equalling the thickness 

 of beds deposited. 



The question as to the age of the formations under discus- 

 sion is the question as to the length and importance of the 

 periods in which the events clearly recorded in their sediments 

 took place. From the standpoint of structural and physical 

 geology an adequate chronology will distinguish or separate 

 this period from that of the coal-bearing Laramie ; but accord- 

 ing to the principles of geology the measure of the geologic 

 time involved is to be sought in the fossils, — in a comparison 

 of the life of the two periods which are to be distinguished. 

 But at the very outset of such an attempt it becomes evident 

 that a very large number of fossils said to occur "in the Lara- 

 mie" are not at present available for such a purpose. This 

 difficulty arises chiefly from the fact that the stratigraphical 

 position of the beds of many localities where " Laramie " 

 fossils have been found has not been at all clearly determined. 



In their recent discussions of orographic movements Messrs. 

 Emmons and Hills have treated the post-Laramie disturbance 

 as that closing Cretaceous time. The former says : "With the 

 exception of the great unconformity between the Archaean 



* The lithological evidence upon which the distinction of the local formations 

 above mentioned has been largely made will probably make little impression 

 upon some readers. It is a striking fact in many observations of the past and of 

 the present in the western region, that the material constitution of coarse grained 

 sandstones and conglomerates has been and is now practically ignored. But if a 

 conglomerate in the apparently conformable Cretaceous section contains pebbles 

 of Niobrara limestones, of the extremely characteristic Dakota conglomerate, of 

 red Jurassic or Triassic sandstone, or of Carboniferous chert, a definite unconfor- 

 mity of great magnitude is made known which does not require proof in visible 

 angular unconformity or in the fossils, though the latter undoubtedly will confirm 

 it when they are known in sufficient detail. 



