sT. JOHN.| TUFACEOUS SPRING DEPOSITS WARM SPRING CREEK. 267 
elevation of, perhaps, 150 feet above the river, it is surmounted by a 
broad, low cone or mound, in the centre of which oceurs the irregular 
orifice of the spring. The spring has long since ceased to overflow, and, 
if it is not extinct, its waters seek subterranean outlet. The orifice has 
a diameter of about 15 by 50 feet and a depth of 25 to 30 feet, with a 
cavernous chamber extending under the southwest end which was par- 
tially filled with still water. The outline and section of this ancient 
spring is shown on accompanying plate. In an irregular bench, about 
250 feet above the last, a still heavier accumulation “of tufaceous lime- 
stone occurs, which for ms the highest deposit observed at this locality. 
The mass forms sloping benches in the mountain declivity, made up of 
thin lamine and layers, 4 to 6 inches thick, of a more compact, dense 
structure than that of the more modern deposits now in process ‘of tor- 
mation. 
Looking up the Warm Spring Cafion an interesting and picturesque 
natural bridge spans the stream, which, although it was not visited, has 
every appearance of the tufaceous rock above noticed. Its origin at 
this locality, wedged in between the precipitous limestone watis, is in- 
volved in some obscurity; at all events the mass has the appearance of 
once having choked the bed of the cafion, the stream subsequently cut- 
ting a channel beneath without undermining the deposit. A short dis- 
tance below the above-mentioned spring orifice, in the east-side angle of 
terrace bluff, another extinct spring mound occurs. The formation, in 
the present instance, is nearly circular in outline and about 45 feet in 
diameter. It forms a low, broad-topped mound rising 8 to 10 feet above 
the level of the terrace, its centre slightly concave, where the orifice is 
completely choked by the dishing depositions that were precipitated in 
the last stages of the spring’s activity when it had ceased to overflow 
the rim of its shallow basin. A partly ideal section of the spring mound 
is given in an accompanying plate. 
About a mile below Warm Spring Creek and half a mile back from 
the river quite extensive deposits of calcareous tufa fill the mouth of a 
ravine in the edge of the terrace, where they form a series of broad steps 
or successive bench-overflows, of which there are three well-marked 
ones. They are being built up by active springs whose temperature 
was about 84° F. On the east side of the ravine a rather prominent 
extinct spring crater rises from the slope, its summit about 200 feet 
above the wide bench descending to the river. ‘The crater, which is 
surrounded by irregular, vertical walls, is about 15 feet in its longest 
diameter by 10 feet in the shorter, and a depth of 20 feet, the bottom 
filled with débris. The thin layers of porous tufa limestone, of which 
the mound is composed, incline gently from the centre, on ail sides, and 
are sparingly intermingled with water-worn pebbles like those found in 
the drift of the terrace benches. The deposit is probably based on the 
sloping terrace, but its contact with the loose, superficial deposits is con-. 
cealed by the gravelly soil surrounding the base of the mound. 
A mile or two below the last locality mentioned above, extensive de- 
posits of tufa limestone occur in the slopes at an elevation 250 feet or 
more above the stream. Its outcrop, facing the valley, forms low blufis, 
in which the regularly bedded and nearly horizontal strata show a thick- 
ness of 50 feet. Seen at a distance the exposures are very deceptive, 
aid without previous knowledge of these occurrences their origin might 
readily be mistaken. The deposit consists of grayish-buff, more or less 
compact and porous, even-bedded calcareous tufa, very gently inclined 
to the west, in one place apparently conforming to the declivity of the 
terrace bench upon which it rests. In texture the rock is firmer and 
