2G EXPLORING EXPEDITION PROM SANTA Vti 



strikingly resemble the fresh-water Tertiary limestones of the Paris basin; and, as I 

 have said, arc andistinguishable from those ofthe "bad-lands" of the Upper Missouri, 

 shown by their fossils to be all of fresh-water or estuary origin The great mass of 

 the Arkansas beds is made up of white or cream-colored limestone, closely resembling 

 much ol the calcareous tufa deposited from springs, and frequently containing masses of 

 black or red, very light and porous scoria : with this tufaceous limestone -are associated 

 strata ol more compact laminated cream-colored limestone 1 : a bed of coarse, friable 

 light-colored sandstone, frequently a conglomerate; and at a higher level a stratum of 

 exceedingly coarse conglomerate, of which the pebbles, if such they can he called, 



are often (! or <S inches in diameter. These pebbles are principally composed of 

 quartz or the harder erupted rocks, basalt, porphyry, &c, with occasionally a frag- 

 ment ol Carboniferous limestone. I noticed that in going toward the west the mate- 

 rials composing these sandstones and conglomerates became much coarser, showing that 

 they had been derived from the direction of the Rocky Mountains. Although the 

 Tertiary basins of the West lane been studied in but a small portion of their extent, 

 and we are as yet very far from being in possession of all the facts in reference to 

 their areas, their structure, <>r their fossils, which will permit us to write in full the 

 history of their deposition, the observations already ma.de all seem to point to the con- 

 clusion that the Tertiary epoch was an era of progressive elevation overall the central 

 portions of our continent ; and that during the greater part of this epoch, the continent 

 had nearly the form and area which it has at present The purely marine Tertiaries 

 appear to be restricted to the immediate vicinity of the present ocean ;| and to a 

 narrow belt along the valley of the .Mississippi, which continued to be occupied till a 

 Comparatively recent period by an arm of the Grulf of Mexico. So far as at present 

 known, all the Tertiary Strata which are found between the Mississippi and the Sierra 

 Nevada are of fre8h-water or estuary origin. The gradual retrocession of the ocean 



is also indicated by the fact reported by Dr. ffayden that where estuary shells are 



found in the Tertiary strata of Nebraska tl icy are generally restricted to the lower beds 



of the series; the .overlying strata containing fresh- Water species. We are led to infer, 

 therefore, that the Tertiary basins which skirt the bases ofthe Rocky Mountains, werecmce 



the beds of rivers and lakes of the Tertiary continent; and. except in the immediate 

 vicinity of the coast-line, were wholly occupied by fresh water. It is also probable 

 that some of these basins occupy former lines of drainage from the Rocky Mountains; 

 and that the beds of course sand and gravel made up ol' fragments of crystalline rocks, 

 wholly foreign to the localities where they are found, but abundant in and peculiar to 

 the Rocky Mountains, were transported from their distant places of origin by the rapid 

 currents of these ancient rivers. The further consideration of these facts, as well as 

 others bearing on the subject of the physical geography of the central portions of the 

 continent during the Tertiary epoch, must be deferred to a subsequent portion of tin's 

 report, where it will more properly find place. 



The details of structure of the Tertiary basin of the Arkansas will lie, perhaps, 

 most readily understood by a few extracts from my notes, made at various points along 

 our route where the Tertiary strata are exposed. "After leaving Pawnee Fork, the road 



passes over level bottom-lands for several miles, where it divides; the left-hand branch 



