2T4 GEOLOGIGAL SURVEY OF THE TERRITORIES. 
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 coy-- 
ered with quantities of vesicular scoriae. The rough and craggy knoll 
immediately overlooking Castello’s Ranch, the reputed 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 divisions 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 of 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 upper 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 
the source of the volcanic ashes must have been close at hand seems 
abundantly proved by the difference in the deposits at the extreme ends 
of the lake, as will be shown in the sections to be given. Not only does 
the thickness of the different beds differ at the two points, but it is dif- 
ficult to bring them into anything beyond the most general concord- 
ance. 
There are still other proofs of disturbance. Around one of the gran- 
itic islands in the southern lake basin the shales mentioned were capped 
by from one and a half to two and a half meters of sedimentary material, 
reaching nearly to the crown of the hill, the lowest bed of which, a little 
more than three decimeters thick, formed a regular horizontal strat- 
um of small volcanic pebbles and sand (A and B of Dr. Wadsworth’s 
note further on); while the part above is much coarser, resembling a 
breccia, and is very unevenly bedded, pitching at every possible angle, 
seamed, jointed, and weather-worn, curved and twisted, and inclosing 
pockets of fine laminated shales, also of voleanic ash, in which a few 
fossils are found (C of Dr. Wadsworth’s note). These beds cap the se- 
ries of regular and evenly stratified shales (D of the same note), and are 
perhaps synchronous with the disturbance which tilted and emptied the 
basin. The uppermost evenly bedded shales then formed the hard floor 
of the lake, and these contorted beds the softer, but hardening, and 
therefore more or less tenacious, deposits on that floor. 
The excavation of the filled-up basin we must presume to be due to 
the ordinary agencies of atmospheric erosion. The islands in the lower 
lake take now as then the form of the granitic nucleus; nearly all are 
long and narrow, but, their trend is in every direction, both across and 
along the valley in which they rest. Great masses of the shales still 
adhere equally on every side to the rocks against which they were 
deposited, proving that time alone and no rude agency has degraded 
the ancient floor of the lake. 
The shales in the southern basin dip to the north or northwest at an 
angle of about two degrees, and an examination of the map will show that 
the southern end of the ancient lake is now elevated nearly two hun- 
dred and fifty meters above the extreme northwestern point. The 
_ greater part of this present slope of the lake border will be found in the 
*Their rude fortifications still crown the summit. 
