THE TERTIARY PERIOD. 235 



of the geosynclinal of the continent to the Missouri and Mississippi 

 valleys, that helped to bring the Pliocene to a close. It is not at all 

 impossible that future investigation will show that the present divide 

 between the Missouri and the Mississippi was a j^ortion of this rim, 

 and that the Pliocene deports that once covered eastern Nebraska 

 were removed by erosion during subsequent glacial times. The 

 Pliocene at least was deposited in a broad level lake between the 

 Meridian of 9S and 105 , and subsequently this whole area of sub- 

 sidence towards the east, accompanied by slight continued elevation 

 towards the west, was transformed into an incline from the base of 

 the foot-hills eastward. " From the 40th parallel region this dip of 

 the Pliocene at present towards the east is equal to 4,000, and to- 

 wards the south of 7,000 feet." — (King). The original discovery of 

 the eastern conditions of the shore line of this old Pliocene lake of 

 the plains was made by Lieutenant (now General) Warren, in the 

 annual report of Captain (now General) Humphreys, for tie year 

 185S. No clearer statements of this theory, and the reasons for it, 

 have since been made. I announced the same theory in public lec- 

 tures as early as 1S72, and had adopted it without being aware that 

 Warren had long anticipated me. Clarence King, also, by inde- 

 pendent study, without knowing of Warren's discovery, had come 

 to the same conclusion. I mention these facts to show that students 

 of geology, in studying the phenomena of this region, will be 

 forced to make this explanation. One curious feature of this sub- 

 sidence of 4,000 feet eastward over the Pliocene region, is that no 

 faults, breaks or crumplings have yet been detected. As the sedi- 

 ments of this old Pliocene lake are thickest next to the mountains, 

 and thin out eastward, it is clear that the eastern rim was a low 

 land, without lofty ridges or mountains. The streams that drained 

 into it from that quarter were of insignificant size. 



Materials of the Pliocene Beds of the Plains. — Near the mountains 

 the materials of the Pliocene beds are exceedingly coarse, and 

 where they are in contact with the foot-hills they are composed of 

 conglomerates made up of water- worn pebbles, feldspar and quartz 

 in masses, and some small pieces or chips of all the Archaean rocks 

 which are represented towards the west. The fragments are of all 

 sizes, from. a shot to a man's head, and even larger. The coarser 

 conglomerates form the upper beds, beneath which there are often 

 much finer materials. The erosion of the upper strata has in many 

 places cut through the coarse conglomerates and widened the bed 

 below in the finer sediments, producing over-hanging rocks. 



