24 GEOLOGICAL SURVEY OF THE TERRITORIES. 



composed of a considerable thickuess of dark iron-rust sandstone, in 

 ■which the plants are found for the most part. In the West this sand- 

 stone is not found in so marked a degree. Volume 7 of the quarto 

 series, by Professor Lesquereux, contains 30 plates entirely devoted to 

 the plants of this group. The physical history of this group would be one of 

 great interest if we had all the details, forming, as it does, the line of 

 separation between two of the most important of the Mesozoic ages. 

 Indeed I have sometimes regarded it as a sort of tro.nsition group be- 

 tween the true Jurassic and Cretaceous series. The exact line of separa- 

 tion between the Cretaceous and Jurassic 1 have never seen, unless it be 

 the lowest layer of sandstone. The beds of sandstones, quartzites, 

 &c., form the characteristic feature of the group, but the beds are 

 usually separated by perhaps layers of indurated clay or shale, which 

 are of variable thickness. It is also probable that the Dakota group 

 passes gradually into the Jurassic through softer strata, for the sand- 

 stones are indurated with lower arenaceous clays. I hold the position 

 that the sequence of all formations is to be sought for in all places ; 

 that while breaks not unfrequently occur, the normal condition is the 

 entire absence of any line of demarkation, so that with the closest 

 scrutiny the geologist cannot tell where one formation ends and another 

 begins. Variability in texture and composition is regarded as indicative 

 of transition from one age to another, and this peculiarity is so persistent 

 in the Dakota group wherever it is known that I have been disposed 

 to regard it as a transition series, although the organic remains do not fix it 

 positively at the base of the Cretaceous. It is even possible that an uncou- 

 formability of sequence will yet be found. All through the group the 

 layers give unmistakable evidence of the shallowness of the waters of the 

 ocean during the deposition of the sediments. It seems somewhat sin- 

 gular that so widely-distributed sea-deposited rocks should exhibit such 

 uniform proof of shallow-water deposition. The rocks are usually in 

 rather thin layers, with very irregular laminse of deposition ; the sedi- 

 ments vary from a fine to a coarse sand or gravel, also from a fine 

 pudding-stone, made up of an aggregate of smoothly-worn jiebbles from 

 the size of a pea to an inch or more in diameter, sometimes so closely 

 cemented together that the fracture of the mass is liable to i^ass 

 through the pebble. In somewhat rare instances this iiuddiiig-stone 

 becomes a coarse conglomerate. The average thickness may be stated 

 at 200 feet, but vibrates between 100 and 250 feet. Between the sand- 

 stones are perhaps partings of indurated arenaceous clay, and about 

 the middle of the group is a seam of impure Lignitic clay, which has 

 often been prospected for coal. Along the Missouri Eiver, in the vicin- 

 ity of Sioux City, Iowa, and in Southern Nebraska and Kansas, this 

 last is 2 to 4 feet in thickness, and has been wrought to some extent 

 for fuel, but with poor success. Along the margins of the mountain- 

 ranges there is no certainty of its appearance at all ; still it crops 

 out from point to point from the north line to Mexico. The entire 

 group of sandstones show that shallow water and land were near, 

 or at least areas where vegetation could grow, for all through the rock 

 are fragments of leaves, stems or sticks, and sometimes coal. Many ot 

 the beds are sort of mud-sandstones, of a drab-brown, from the a,bun- 

 dance of indistinct fragments of vegetable matter. Sometimes, in the 

 more compact homogeneous layers, well-defined leaves are found, usually 

 of a deciduous type. Leaves were found in these rocks, near Denver, 

 along the line of the Union Pacific Eailroad to Ogden, and in the Elk 

 Mountains. The sandstones pass up into brown arenaceous clays, with 

 rather thin layers of mud-sandstones, full of mud-markings, which re- 



