SURVEY OF COLORADO AND NEW MEXICO. 13 



tain a considerable thickness, but southward thin out to a few feet, or 

 are entirely concealed by the debris which everywhere prevails. 



These red beds, when they make their appearance, often give the most 

 unique and remarkable features to the scenery, and any development of 

 them, however small, never fails to attract even the commonest observer 

 on account of their brick-red appearance. JSTo well-authenticated fossils 

 have ever been found in them, yet they are regarded as of triassic age 

 by the common consent of geologists. I am inclined to believe that a 

 portion of the upper light-red beds, with the included layers of flinty, 

 limestone, are Jurassic, but I have never been able to find any well de- 

 fined line of separation between what are well known to be Jurassic and 

 the supposed triassic beds. 



Besting above these red beds is a series of marls and arenaceous marls 

 of a light or ashen gray color, with harder layers of limestone or fine 

 sandstone, which were also first discovered around the margin of the 

 Black Hills of Dakota in 1857. Since the discovery in the Black Hills, 

 Jurassic fossils have been found over a very wide geographical area, and 

 yet I have never seen them so well developed, or the peculiar fossils so 

 abundant, as at the locality where they were first observed. Although 

 I have traced this Jurassic belt by its organic remains over many hun- 

 dreds of miles, I have been able to discover scarcely a well-defined 

 Jurassic fossil south of Deer Creek, a point one hundred miles north of 

 Fort Laramie, or south of the Lake Como, on the Union Pacific railroad. 



I believe that a thin remnant of this belt extends tar south to New 

 Mexico, but it is often so obscured, or so easily concealed, that I have been 

 continually in doubt in regard to its existence. Coextensive with all 

 the mountain ranges is a large series of beds above the Jurassic belt 

 which belong to the cretaceous period, the upper and middle portions of 

 which are everywhere indicated by characteristic fossil remains, as seen 

 on the Missouri Eiver, where they were first studied by Mr. F. B. Meek 

 and the writer. The cretaceous rocks present five well-marked divisions, 

 Nos. 1, 2, 3, 4, and 5, or Dakota group, Fort Benton group, Mobrara 

 division. Fort Pierre group, and Fox Hill beds. On the Lower Missouri 

 IsTo. 1, or Dakota group, is characterized by several species of marine 

 shells and a profusion of impressions of deciduous leaves; but along the 

 margins of the mountain elevations I have never been able to discover 

 a single specimen of organic remains that would establish the age of the 

 rocks. I only know that there is a series of beds of remarkable persist- 

 ency all along tlie margin of the mountain ranges, holding a position 

 between well-defined cretaceous No. 2 and Jurassic beds, and in my ]3re- 

 vious reports I have called them transition beds, or No, 1. They consist 

 of a series of layers of yellow and gray, more or less fine-grained sand- 

 stones and pudding-stones, with some intercalated layers of arenaceous 

 clays. In almost all cases there is associated with these beds a thin series 

 of carbonaceous clays, which sometimes becomes impure coal, and con- 

 tains masses of silicified wood, &c. On the west side of the Black Hills 

 they assume a singularly massive appearance, neai'ly horizontal, two 

 hundred to two hundred and fifty feet thick, and are called Fortification 

 Eocks. Here also occurs a thin bed of carbonaceous clay. On the east- 

 ern slope of the Big Horn Mountains I observed this same series of beds 

 in the summer of 1859, holding a position between cretaceous No. 2 and 

 the Jurassic marls, with a considerable thickness of earthy lignite, large 

 quantities of petrified wood, and numerous large uncharacteristic bones, 

 which Dr. Leidy regarded as belonging to some huge saurian. 



There are very few points of resemblance between these beds aiid 

 those which form the Dakota group, as seen in Kansas and Nebraska. 



