SURVEY OF COLORADO AND NEW MEXICO. 83 



ridges are all so grassed over tliat the true nature of the underlying- 

 rocks is not easily determined. Then comes ridge after ridge until aU 

 the beds — -Jurassic and cretaceous — are shown. 



On this stream we have a fine system of terraces. On the north side 

 are three distinct terraces above the bottom, and the lowest one has a 

 bed of cretaceous sandstone, nearly horizontal, cropping out at its base. 

 This is a low one, not more than fifteen feet high; the next one is fifty 

 feet high, and the third, which descends from the high hills, is two hun- 

 dred feet. A little west of south, at the junction of Grand Kiver with 

 Eraser Creek, five high i)eaks are visible, which form in that direction 

 a part of the main range. All around us, in every direction, we could 

 see the snowy peaks, and the beds which form the ridges of upheaval 

 iuclining in every direction. 



To the south of the park the older sedimentary rocks dip north in 

 lofty ridges, at least two thousand feet high, jiresenting high escarp- 

 ments when split by streams, and reaching almost the highest margin 

 of the mountains. 



About ten miles above the hot springs, Grand Eiver flows through an 

 enormous gorge cut through a high ridge of basalt, which seems to be 

 an intrusive bed, for above and below, the sedimentary rocks are well 

 shown, but partially changed. Underneath are the cretaceous shales of 

 i>ros. 4 nnd 5, and above are the lignite tertiary beds. These beds all 

 dip west twenty-three degrees. 



These eruptive rocks are very rough, as if they had been poured out 

 Avitliout much pressure. Much of it is a very coarse conglomerate, the 

 inclosed masses appearing to be the same kind as the paste; that is, orig- 

 inally, of igneous origin. Some of the inclosed rocks are very compact, 

 close, and all wert , more or less worn before being inclosed. This rock 

 is a true dolorite. I did not see any inclosed masses that I could call 

 unchanged. This basalt extends a great distance, continuing a nearly 

 uniform thickness, and inclining in the same direction with the cretaceous 

 beds below and the tertiary beds above. 



On both sides of Grand Eiver, but especially on the east and north- 

 east sides, extending up nearly to the foot of Long's Peak, are quite 

 large exposures of the recent tertiary beds. They are nearly horizontal, 

 and have much the appearance in color of the Fort Bridger beds, of 

 which Church Buttes is an example. These beds are composed, for the 

 most part, of fine sand and marl, but there are a few small rounded 

 boulders scattered through it. Below the gorge, on the north side of 

 Grand Eiver, these outflows of basalt have formed some well-defined 

 mesas; at least three beds ascending like steps from the river. Below 

 the gorge the river flows through what seems to be a rift of basalt, that 

 is, on the north side. The basalt lies in horizontal beds, but on the south 

 side is the sloping side of a basaltic ridge. The dip is nearly northwest, 

 though the trend of this basaltic ridge is by no means regular. One 

 portion of it has a strike northwest and southeast, and another north 

 and south. The tertiary rocks reach a great thickness, and are elevated 

 high up on the top of the basaltic ridge, eight hundred to one thousand 

 feet above the river. They are mostly formed of fine sandstone and i)ud- 

 ding-stone. These fine sandstones contain some well-marked impres- 

 sions of deciduous leaves, among which are good specimens of Platanus 

 haydeni. On the north side of Grand Eiver, in some localities, the tertiary 

 beds are elevated so high, on many of the eruptive mountains, that they 

 are covered with perpetual snow. These eruptive beds are certainly 

 among the most remarkable examples of the overflow of igneous matter 

 that I have ever seen in the West. 



