.,.„,^-,-i GEOLOGY ERUPTIVE EOCKS OF THE PLAINS. 129 



der, lying some 800 feet below. Opposite rise the high flat-topped hills, 

 composed mostly of the nearly horizontal lignitic strata, with the Mar- 

 shall and other mines upon their sides, but capped with the same gravel 

 deposits. 



iSouth of the Boulder Creeks to Coal Creek the long sweeps of high 

 gravel terraces are quite continuous, and reach quite up to the higher 

 slopes. Then they again diminish io altitude. From North Table Mount- 

 ain, near Golden City, they may be seen at the north extending in two 

 or three terraces from the higher hog-backs tar eastward to near the 

 junction of Clear Creek with the South Platte, a few miles below Denver. 

 Kear this point several i^rominent outlyers occur, rising a few hundred 

 feet above the river, and capped with from a few to 50 or 60 feet of 

 siliceous gravel. Even to the east of the Platte a few remnants are 

 found still retaining their cap of archsean debris, chroniclers not only.of 

 the former wide extent of the bed, but of the great amount of erosion 

 which has taken place since it was deposited. Over a great deal of this 

 region patches of debris from this bed may be found, the remains after 

 the weathering away of the soft subjacent strata. Farther south, from 

 near Bear Creek to the Platte, the terraces are again lower, and much 

 like those at the far north. 



It is here hardly necessary to refer to the latest of all the geological 

 formations, and that progressing at the present time, the alluvium 

 which has accumulated in places along all the larger streams of the 

 plains. In places spreading out in considerable areas, it forms, both on 

 account of natural fertility and ease of irrigation, (which has here to be 

 resorted to,) some of the best agricultural lands in the Territory. Oth- 

 erwise it is of no special interest, being apparently like most local river 

 alluviums. 



THE ERUPTIVE ROCKS OP THE PLAINS. 



The eruptive rocks east of the mountains are exceedingly limited in 

 extent, and would barely attract any attention but for the fact that 

 they occur in the midst of thickly-settled regions. The only points at 

 which they are known are at Valmont, situated at the junction of the 

 North and South Boulder Creeks, and near Golden City. At the latter 

 place are the two well-known table-mountains which form such con- 

 spicuous objects in the foreground of the high range when viewed from 

 the east. The accompanying sketch (Fig. 5) by Mr. Holmes gives their 

 appearance as viewed from Bear Creek Station, a high i)oint on the 

 very border of the mountains overlooking the plains, and about six 

 miles south of Golden City. The characteristic appearance of the i^lains 

 to the eastward is also well given. The table-mountains are separated 

 from the granite mountains by a valley about a mile in width, in which 

 Golden is situated, and stand one on either side of the gorge through 

 which Clear Creek flows after debouching from the mountains. The 

 railroad now passes through this gorge and on up the caiion of Clear 

 Creek to the mining regions. These two hills, which are irregular in 

 shape, with diameters but little over a mile in any direction, are formed 

 below of the horizontal strata of the lignitic group, capped by layers of 

 basaltic lava. The northern table stands the highest, reaching some 

 900 feet above Clear Creek, and 700 or 800 above the surrounding valley. 

 The tops of both are nearly in the same plane, which dips gently to the 

 south and east at an angle of about two or three degrees. The source 

 of this lava is from beneath North Table Mountain, on the summit of 

 which, and near the northwest corner, the remnants of a group of small 

 volcanic cones may still be seen; weather-beaten, and nearly worn away, 

 9 G s 



