Art. LXVII.—Notes on the Chemical Properties of some of the Strata from Mr. Firth's Well at Mount Eden.
[Read before the Auckland Institute, 21st September, 1874.]
In bringing this subject before the Society, it was not so much with a view of adding largely to the evidence relative to the constituents of the strata in question, as to keep alive the interest exhibited by the fact of the discovery of vegetable markings two hundred feet below the base of Mount Eden. In considering the different strata gone through, I have confined my attention chiefly to those marked d and c on Mr. Firth's plan, * as the c stratum contains the earliest vestiges of organic impressions, and the difference in colour between the adjoining earth led me to choose these two more prominently.
The samples in question differ slightly in their chemical composition, the following being the analysis of d and c samples:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Sesquioixide of iron||12·1|
|Chloride of sodium||a trace|
|Sesquioixide of iron||17·4|
|Chloride of sodium||a trace|
The excess is owing to protoxide of iron in small quantity in both samples reduced to peroxide.
[Footnote] * See table at end of Art. LXXV.
The samples are meagre in a chemical point of view, the chief difference being in the disproportion in alumina and lime in the two samples.
When viewed microscopically a marked distinction will be found between the lowest deposit and the upper ones, as the particles of the one in question are angular, sharp edged, and in a large measure pure fragments of silica interspersed with argillaceous earth with peroxide of iron; the others, on the contrary, are composed of abraded grains of siliceous sand permeated with hydrated peroxide of iron, and most probably placed in their present position by the action of water.
This seems peculiarly confirmatory of Mr. Firth's conclusion relative to this portion of the strata having been at one time the bottom of a lake; the d stratum of which having been formed by an upward flow of molten lava coming in contact with sand containing bisulphide of iron, found all through this part of New Zealand, would have the effect as noted, while the other strata would take some time to form, by the action of water, to their present thickness. The b sample is undoubtedly laid under water, and contains patches of obsidian and other volcanic matter distributed arbitrarily throughout. This and the a deposit contain the greatest traces of organic markings, though the latter is less crushed and displaced than the former, leading to the inference that the upper stratum had become hard and firm, probably in the hot summer months before another eruption entirely covered it with volcanic ash.
In the cylindrical holes in samples a and b, which have undoubtedly contained stems and roots of plants, I found a white deposit where uncontaminated by adhesion to the sides of the holes, when it had a brownish hue. On examination I found the white portion to be pure silica tinged at the junction with the side with peroxide of iron. In no instance could I detect any vegetable formation with the microscope in the deposit, but simply rough granular cohering particles; nor were there in any of the strata, so far as I could note, any evidences of organic matter other than that already referred to.
In reference to the amount of siliceous matter in the tubes, I would add that I know of no plant with the same diameter of stem that could deposit so large an amount of silica, certainly not the raupo, but this is not necessary to be proven in this instance. Nor would the excess of this base disprove the vegetable origin of the holes, as silica is volatile in the superheated vapour of water, and is deposited during condensation of the steam. This would probably account for the excess of the deposit, as the volcanic matter falling so close to the place of its ejection would fall upon the sandy soil, which, no matter how dry the land might be, would, in the presence of vegetation, obtain sufficient water to volatilise and deposit silica in the orifices that had originally contained the stems of the plants. One other point worth
mentioning is the increase of lime in the upper beds, pointing to a gradual infiltration from some of the volcanic beds of later deposition.
I cannot help thinking that this subject sadly needs the attention of some of our geologists more particularly, and that in so doing a rich field would be brought before them. Dr. Hochstetter, speaking on extinct volcanoes, and particularly of this locality, instances the rising of Monte Nuovo, near Naples, which in two days and nights grew to the height of 400 feet, and remarks, “we may venture to say that cones, such as Mount Eden, are likely to have sprung up in the course of a few days.”
Now, from Mr. Firth's researches we have seen that this volcano, when in action, must have been intermittent to have allowed time for the deposition of the vegetable-bearing strata b and a, with the volcanic ash between them, and for the clothing the same on each occasion with vegetation before its instant destruction by the hot débris. But with respect to the several eruptions later than this period, and mentioned in Mr. Firth's paper to the number of five, I think it is very probable that they will, on examination, resolve into a smaller number of perhaps very short duration, as in the presence of so many craters, and in this instance so close to the base of the largest, enormous masses would be ejected, taking the form of scoria, ash, or mud, as the absence or presence of water with the consequent steam and terrific pressure would determine. To decide this question, however, great care would have to be exercised in the examination of the surfaces of the different strata, as in many instances minerals when first ejected in volcanic matter are in the state of sulphides or sulphurets, which, by exposure to the action of air and water for any length of time would become oxidized, and, in the presence of iron, immediately discernible. By this means a very close approximate to time could be made in many instances, giving reliable data as the basis for future calculations on this very interesting subject.