SCUDDER. ] TERTIARY LAKE BASIN OF FLORISSANT. 2713 
is Pike’s Peak; to the west South Park and the caion of the South 
Platte, shown by a depression ; to the extreme south, the grand canon 
of the Arkansas; while to the north a few sharp, ragged, granite peaks 
surmount the low wooded hills and ravines characteristic of the nearer 
region. Among these hills and ravines, and only a little broader than 
the rest of the latter, lies, to the south, the ancient Ilorissant Lake 
basin, marked by an irregular L-shaped grassy meadow, the southern 
balf broader and more rolling than the northwestern, the latter more 
broken and with deeper inlets. 
Recalling its ancient condition, and it will appear that this elevated 
lake must have been a beautiful, though shallow,* sheet of water. 
Topaz Butte, and a nameless lower elevation lying eight kilometers to 
its southwest, and which we may call Castello’s Mountain, guarded the 
head of the lake upon one side and the other, rising three or four hun- 
dred meters above its level. It was hemmed in on all sides by nearer 
granitic hills, whose wooded slopes came to the water’s edge; some- 
times, especially on the northern and eastern sides, rising abruptly, at 
others gradually sloping, so that reeds and flags grew in the shallow 
waters by the shore. The waters of the lake penetrated in deep inlets 
between the hills, giving it a varied and tortuous outline; although 
only about sixteen and a half kilometers long and very narrow, its mar- 
gin must have measured over seventy kilometers in extent. Still 
greater variety was gained by steep promontories, twenty meters or more 
in height, which projected abruptly into the lake from either side, nearly 
dividing it into a chain of three or four unequal and very irregular open 
ponds, running in a northwest-southeast direction, anda larger and less 
indented sheet, as large as the others combined, connected with the 
southwesternmost of the three by a narrow channel, and dotted with 
numerous Jong and narrow wooded islets just rising above the surface. 
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 Rocky 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; 
aud 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 Ranch; 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 
* The shallowness of the lake is indicated by the character of the fish, the sun crack- 
ing of some of the shales, and the erect sequoia stumps. 
18 H 
