26 EXPLORING EXl'KIMTION FROM SANTA Fft 



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

 have said, are undistinguishable from those of the "bad-lands" of tin- l T pper Missouri, 

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

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

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

 black or red, very light and porous scoria; with this tiiiaceous limestone are associated 

 strata of more compact laminated cream-colored limestone; abed 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 be called, 

 are often 6 or 8 inches in diameter. These pebbles are principally composed of 

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

 ment of Carboniferous limestone. 1 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 Ivocky Mountains. Although the 

 Tertiary basins of the West have 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, or their fossils, which will permit us to write in full the 

 history of their deposition, the observations already made 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 Gulf of Mexico. So far as at present 

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

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

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

 found in the Tertiary strata of Nebraska they are generally restricted to the lower be< Is 

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

 therefore, that the Tertiary basins which skirt the bases of the Rocky Mountains, were once 

 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 Rockv Mountains; 

 and that the beds of coarse sand and gravel made up of 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 this 

 report, where it will more properly find place. 



The details of structure of the Tertiary basin of the Arkansas*will be, perhaps, 

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

 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 



