GENERAL CHARACTER OF BLACK HILLS 385 



miles in breadth, with its larger dimension lying nearly northwest and 

 southeast. It is situated in the great east-sloping plain that extends 

 from the Rocky mountains to the Mississippi river. It has brought 

 above the general surface level an area of pre-Cambrian crystalline 

 rocks, now eroded into a small group of mountains, on the flank of 

 which is exhibited a complete sequence of the Paleozoic and Mesozoic 

 rocks from Cambrian to Laramie, all dipping away from the central 

 nucleus. The region is one of exceptionally fine exposures, which afford 

 rare opportunity for a study of stratigraphic relations and variations. 

 The Jurassic beds are seen here for the first time as one goes westward 

 across the United States, and present many features of interest, some of 

 which will be described in the following pages. 



Previous Investigations of Black Hills Jurassic Beds 



The existence of Jurassic beds in the Black hills was first ascertained 

 by Hayden. who, in 1857, discovered marine fossils which were identi- 

 fied and described by Meek. Hayden's descriptions of the Jurassic 

 beds were meager, and he included in the Jurassic the Red beds, and at 

 one point a limestone with fresh-water fossils which I have recently 

 found to be Tertiary. 



N. H. Winchell visited the region in 1874, and in his report recorded 

 a few general facts as to character and distribution of some of the Juras- 

 sic beds along his line of travel. 



Henry Newton made a very much more extended survey of the Hills 

 in 1875, and in his report added greatly to our knowledge regarding the 

 Jurassic deposits. He described a number of typical exposures in con- 

 siderable detail, but attempted no classification of the members. His 

 principal statements are as follows : 



" The Jura of the Black hills consists primarily of gray or ash-colored clay or 

 marls, with occasional bands of green and red. Interbedded with these are soft 

 sandstones more or less argillaceous and a few restricted bands of limestone. . . . 

 The thickness . . . is about 200 feet, but it shows a remarkable increase 

 towards the north and northw^est, attaining in the Belle Fourche valley a depth 

 of nearly 600 feet. . . . On the north and west and to a less degree to the 

 south the formation is well exposed and characterized by a greater or less abun- 

 dance of fossils. On the southeast and east it is less plainly seen, being usually 

 covered by a broad talus, and, so far as examined, it was not found to be fossilif- 

 erous. ... It was found impossible to base a subdivision of the formation 

 either on persistent lithological characters or on the distribution of fossil forms. 

 .'' . . The Jurassic strata . . . are always easily distinguished from the Ked 

 beds. . . . Everywhere a large portion of the formation is composed of sand- 

 stones which are usually light in color and even sometimes a snowy whiteness." 



