108 GEOLOGICAL SURVEY OF THE TEKRITORIES. 



the upturned edges of the strata all the lower formations are found dipping 

 eastward at an angle seldom above 60°, and generally much less, until, 

 upon reaching the Upper Cretaceous and lower lignite beds, they are 

 found — at least all through the middle of the district — to be tipped up 

 nearly vertical, or even pushed over beyond the vertical, so that they 

 appear to dip at a high angle toward the mountains. 



Immediately there follows an abrupt bend, which at once throws all 

 the remainder of the lignitic series into nearly its natural horizontal po- 

 sition, and thus it stretches eastward as the formation directly underly- 

 ing and forming the plains. All through the plain portion of the dis- 

 trict these nearly horizontal beds have been eroded by the streams into 

 a rolling country, the irregularities of which are really quite considera- 

 ble, the hills frequently rising several hundred feet above the numerous 

 intersecting streams, and, though generally gently molded in long, 

 rounded slopes, still abrupt and irregular bluffs, formed of the edges of 

 sandstone strata, and of considerable height, occasionally occur. Trees 

 occur only along the streams — such as the cottonwood and willow — the 

 hills being clothed in a sort of grassy vegetation of very uniform yellow- 

 green color, so that, standing upon an elevation, all the surrounding 

 hills are thrown against their neighbors with but little sense of relief, 

 and all appears quite level, notwithstanding the decided unevenness of 

 the surface. Along the South Platte Eiver, near and below Denver, 

 the strata dips very gently from either side toward the river, forming 

 a fiat synclinal of the valley. To the east, as the surface gradually rises, 

 pines in scattered groups begin to occupy the country, the precursors 

 of the tract known as the pineries. 



The rocks comprising the lignitic formation, which directly underlies 

 this country, are principally sandstones and clays. As compared with 

 the older formations, these generally present a much newer or less con- 

 solidated appearance. The sandstones are usually of somber colors ; 

 dull reddish-brown and yellow, and rusty gray, perhaj^s, prevailing. 

 They vary from the conglomerates to quite fine sandstones, and are 

 usually friable, and but seldom compact and firm enough for building 

 IJurposes. Some few clean, gritty, white siliceous sands occur. 



The clays which i^redominate in the formation are of the nature ot 

 fire-clays. They are of various colors, most often perhaps light-gray, 

 generally rather soft, and M^hen very fine and unctuous to the touch are 

 commonly called " soapstones." At Golden City excellent fire-brick, pot- 

 tery, &c., are made from some of the lower clays. The transitions be- 

 tween sands and clays are sometimes gradual, giving zones of mixed 

 material; soapstones becoming gritty, and sands argillaceous. No 

 limestones occur, and argillaceous shale and slate only in very subordi- 

 nate quantities. 



Fossil leaves, and fragments of wood, &c., are very numerous in some 

 horizons, usually near the coal. They are mostly of deciduous trees, 

 many of which are closely allied to species common in our Southern 

 States and the tropics, the flora of which this ancient vegetation must 

 have somewhat resembled, while no true coal-measure (Carboniferous) 

 plants, such as ferns, &c., have been found. The remains of trunks of 

 trees of enormous size are sometimes met with. 



The following sections are given as illustrating the general characters 

 of the series : The first, which shows the strata as they occur at Golden 

 City, is from a section made by Mr. E. L. Berthoud, the positions of the 

 outcrops being given on Plate II, section 13. It probably shows a 

 greater portion of the full thickness of the lignitic than exposed else- 

 where, for which we have to thank the j)rotective action of the cap of 



