10 GEOLOGY OF THE TERTIARY FORMATIONS OF 



fresh-water lakes which we now know was a most prominent feature in the physical 

 geography of this country during the Tertiary period. To obtain a clear idea of the 

 plan of growth of the western portion of our continent, as it quietly and slowly 

 emerged from the ocean, we have but to study the numerous barometrical sections 

 which have been constructed by the U. S. Army officers and others for the past 

 twenty or thirty years. Taking almost any point along the Missouri River below 

 Council Bluffs, we find that as we proceed westward there is a gradual elevation or 

 ascent of about one foot to the mile for the first hundred miles, then three feet for 

 the second, five feet for the third, eight or nine feet for the fourth, &c., until at the 

 foot of the mountains the ascent becomes eighty or ninety feet to the mile. We then 

 pass over a series of mountain ranges of different elevations until we reach the west- 

 ern or Pacific slope, when we gradually descend into the ocean. We thus conclude 

 that during the Cretaceous period there was a gradual slow elevation of the whole 

 country west of the Mississippi ; that about the close of that period the crust of the 

 earth had been strained to its utmost tension, and long lines of fracture commenced, 

 which formed the nucleus of our present mountain ranges; for the evidence seems to 

 indicate that there was a long period of quiet elevation, the central force acting 

 along the lines of upheaval. The barometrical profiles seem to indicate that the 

 west forms a vast plateau, upon which are located a great number of ridges or moun- 

 tain ranges, tending in the aggregate nearly north-west and south-east. At the close 

 of the Cretaceous period, or in the early part of the Tertiary, when the crust had 

 been elevated to its utmost tension, it broke, sometimes in long lines of fracture, 

 which gave birth to these lofty continuous ranges with a granitoid nucleus along the 

 eastern portion of the Rocky Mountains, as the Wind River, Big Horn, Laramie 

 Mountains or the Black Hills, or the basaltic ridges, which are less regular in their 

 structure, formed by outbursts of melted matter arranged in a series of sharp peaks, 

 or sierras, as they are called in the Spanish countries, of which the Wasatch, Green 

 River Mountains and numerous ranges on the Pacific coast are examples. 



That the Tertiary beds were in part deposited before the upheaval of the mountain 

 ranges seems to be indicated by the fact that the lignite beds inclined from both 

 sides of the Wind River Mountains, which forms the main axis of elevation. There 

 are many other proofs Avhich tend to show that the first important crust movements 

 must have been during the early part of the Tertiary period, or at least during the 

 deposition of those beds of passage or transition between the well-defined Cretaceous 

 and Tertiary. 



The lowest beds of the Tertiary exhibit a somewhat brackish or estuary character, 

 and a few fossils are found which are peculiar to such waters. Along the foot of the 

 mountains in Colorado, where the lowest Tertiary beds are exposed, a species of 

 oyster is found in the greatest abundance ( 0.^trea subtrigmalis), which appears to be 



