The rocks are entirely different in appearance and structure, that of
the mound being twice as hard as that of the bluff. The former contains
much organic matter as is shown by blackening when it is heated in a
tube and giving off the characteristic odor. The iron is practically
of the ferrous variety, probably combined with carbonic acid, and the
rock contains traces of chlorides. The particular sample taken was at
some distance from the spring, and had been thoroughly exposed to the
weather.
The rock of the mound is of just such a character as might have been
built up by deposition from the water, as it contains the least soluble
constituents of the water. The process of solidification would have
been assisted by the silica in the water, forming insoluble cementing
silicates, as noticed by Prof. Patrick. The analysis given above shows
that there is abundant silica in the water for this purpose.
Mention has been made of the organic growth in the adjacent springs.
The mixed scum on being heated changes from a dull green to a vivid
grass-green, and if ignited it swells up and emits an ill-smelling
vapor, which is evidently nitrogenous in its character. A grayish white
ash is left, which contains much carbonate of lime. This is evidently
freshly deposited, as it is entangled in the algæ in granular lumps.
A specimen of the white scum, noticed above, only slightly mixed with
the green algæ, was analyzed. The acid solution of the ash contains
1.26 per cent of soluble silica. This was of course a combined silica,
probably calcium silicate, which becomes the cementing material in
the rock. In another sample of ash, after removing all the substances
soluble in hot water, the residue was found to contain 76.46 per cent
of silica.
The siliceous residue from the scum was examined by Dr. S. W.
Williston. It consists mostly of diatoms. He recognized
Navicula— 2 species
Nitzschia— 2 species
Asteronella— 1 species.
All three genera are found both in fresh and salt or brackish water.
The green material consists essentially of Oscillaria and Confervæ.
If the scum is allowed to stand for a short time a very strong
sulphuretted odor is developed, strangely suggestive of salt water
marshes or mud flats; and indeed the same odor is noticed in the
vicinity of the spring. No characteristic salt water organisms, that
should occasion this peculiar odor have, however, yet been observed
here. A more extended and special study of the organic life of these
interior salt water marshes and springs would be of great interest.
Footnotes:
[5] Mostly FeO, and so calculated.
[6] With alkalies.
On Pascal’s Limaçon and the Cardioid.