282 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [ra.ri. 



The ancient outlet of the whole system was probably at the southern 

 extremity ; at least the marks of the lake-deposits reach within a few 

 meters of the ridge which now separates the waters of the Platte and 

 Arkansas ; and the nature of the basin itself, the much more rapid de- 

 scent of the present surface on the southern side of this divide, with the 

 absence of any lacustrine deposits upon its slopes, lead to this conclusion. 

 At the last elevation of the Eocky Mountain chain, the drainage flow of 

 this immediate region was reversed ; the elevation coming from a south- 

 erly or southeasterly direction (perhaps from Pike's Peak), the lake, or 

 series of lakes, was drained dry by emptying at the northwestern ex- 

 tremity. The drainage of the valley now flowed into a brook which fol- 

 lowed the deeper part of its former floor, and the waters of the region 

 have since emptied into the Platte and not the Arkansas, passing in 

 their course between Topaz Butte and Castello's Mountain. 



The promontories projecting into the lake on either side are formed of 

 trachyte or other volcanic lavas, apparently occurring in fissures directly 

 athwart the general course of the northwestern or upper series of lakes; 

 and masses of the same occur at many different points along the ancient 

 shore, such as the western corner where the waters of the lake were 

 finally discharged; in the neighborhood of Castello's Eanch ; along the 

 eastern wall of the lowermost of the chain of upper lakes, near where 

 the present road divides ; and at points along both eastern and western 

 walls of the lower southern lake. In general the trachytic flows seem 

 to be confined to the edges of the lacustrine basin, but some, if not all, 

 of the mesas or ancient islands of the southern lake have trachytic flows 

 over them, and toward the southern extremity of the lake a larger island 

 will be seen upon the map, now forming a rounded hill with steep north- 

 ern walls, crowned by heavy beds of dark trachyte, and its slopes cov- 

 ered with quantities of vescicular scoriae. The rough and craggy knoll 

 immediately overlooking Castello's Eanch, the reiDuted scene of Indian 

 combats,* was witness of hotter times than those ; vertical cylindrical 

 holes, with smooth walls, in which a man could hide from sight, funnels 

 scored by heat, mark, perhaps, the presence of former geysers ; the ba- 

 saltic rocks themselves are deeply fissured by the breaking up of the 

 planes of division between the columns, affording the best protection to 

 the Ute and Arapahoe warriors. But the very shales of the lake itself, 

 in which the myriad plants and insects are entombed, are wholly com- 

 posed of volcanic sand and ash; fifteen meters or more thick they lie, in 

 alternating layers of coarser and finer material. About half ot this, 

 now lying beneath the general surface of the ground, consists of heavily 

 bedded drab shales, with a conchoidal fracture, and totally destitute of 

 fossils. The ujjper half has been eroded and carried away, leaving, how- 

 ever, the fragmentary remains of this great ash deposit clinging to the 

 borders of the basin and surrounding the islands ; a more convenient 

 arrangement for the present explorer could not have been devised. That 



* Their rude fortifications still crown the summit. 



