Art. LXXV.—Deep Sinking in the Lava Beds of Mount Eden.
[Read before the Auckland Institute, 26th August, 1874.]
Early in 1873 I commenced sinking a well in my grounds at Mount Eden, with the view of obtaining a constant supply of water. I was led to undertake this work from the circumstance that at various points around the mountain springs of excellent water are met with. The most notable of these is the spring found by Mr. Seccombe, yielding in the driest seasons about 80,000 gallons daily of most excellent water.
I do not know that a record of my explorations will present any features of much interest to the general public. To myself, though unsuccessful in my search for water, they were full of interest, for, as I penetrated each successive lava stream, it seemed like the turning over of the leaves of some ancient and unknown book. I am disposed to believe I have obtained a few facts which may, in the hands of scientific men, be of some little use in helping them to elucidate some of the phenomena which in bygone times have played so great a part in changing the features of the land we live in. The point of commencement was 329 feet above sea level, and 313 feet below the summit of the mountain. The depth to which I penetrated was 212 feet, or within 117 feet of sea level at high water mark, or about thirty feet below the bottom of Mr. Seccombe's well before referred to.
The accompanying section presents the thickness, position, and details of the successive beds of volcanic ash and lava through which I passed:—
Seventh and latest eruption.
|Brown soil and scoria stones intermixed||12||0|
|Blue scoria rock with patches of quartz.||18 feet||6||0|
|Loose scoria stones with crevices from which issued blasts of cold air; stones covered with soft mud||12||0|
|Loose red scoria ash with round nodules of very dense scoria covered with cement deposits; steel grey fracture, showing abundance of olivine and brilliant coloured crystals||8||0|
|Hard blue rock with patches of quartz||4||0|
|Solid blue rock, close-grained, basaltic.||50 feet||8||0|
|Loose boulders in coarse red scoria ash, pieces of quartz imbedded; full of crevices, through some of which came strong currents of pure cold air. Stones and ash filled with moisture||42||0|
|Hard blue rock.||101 feet||9||0|
|Red and sulphur yellow ashes and very porous rock||4||0|
|Very close-grained hard blue rock||12||0|
|Stratified basaltic rock, dipping from S. to N. at an angle of 30°||6||0|
|Solid blue rock with quartz||132 feet||9||0|
|Red scoria ashes and rough clinker cinders||36||0|
|Hard bluish rock||4||0|
|Hard close-grained rock—Indian red colour||2||0|
|" " "Blue colour.||176 feet||2||0|
|a. Baked clay with round holes in which fern roots had been imbedded||1||0|
|Fine scoria gravel||0||6|
|Red hard scoria, much honeycombed||2||0|
|Hard close-grained blue rock.||180 feet 6 inches||1||0|
|b. Volcanic mud, full of holes (the impress of timber decayed)||2||0|
|c. Soft sandstone rock of light yellow colour, stratified||1||6|
|d. Ochre, like Venetian red of a bright vermilion colour||1||6|
|Black scoria rock, intensely hard, full of air-bubbles or honeycombed. For 2 feet intersected by seams or joints transverse and vertical. These seams were filled with the vermilion-coloured earth. This rock presented every appearance of having been deposited under|
|water, and had contracted in every direction, showing cracks as above described. This black rock continued without any change to the furthest point reached, viz., 212 feet from the surface||26||6|
At this point, there being no indications of a change, nor any signs of water, I brought my explorations to a close.
I have arrived at a few conclusions, which I submit with diffidence, yet, as they are based upon evidence carefully noted as the explorations proceeded, it might be unwise to omit them from this paper. In noting these conclusions, and such evidence as may be necessary, I shall, for obvious reasons, commence at the earliest eruption and ascend to the latest.
The lowest lava bed penetrated 26 feet 6 inches, and presented strong indications of having been a submarine eruption. Upon this rock there appears to have been deposited(at d) a substance like Venetian red. This, I think, had been deposited under water. Whilst in this position it had apparently been washed into the numerous joints or seams to the depth of several feet, which I have described as appearing in the upper surface of the lowest lava stream. Upon this red stratum there lay, dipping at an angle of about 45°S., or towards the mountain, 18 inches of a soft sandstone rock of a light yellow colour (c), having a well-defined stratification, as though the formation had been deposited under water. Though this deposit on analysis showed a trace of chloride of sodium, I am inclined to think it was deposited in fresh water, as if a fresh-water lake had been formed in the ancient crater subsequent to its elevation above sea level. I am the more disposed to advance this opinion, because I have been unable to detect any traces of marine shells or plants in this formation, whilst I found in it some extremely beautiful and well-defined impressions of leaves of toetoe (Arundo conspicua) or of raupo (Typha latifolia). Both these plants love moist ground; the latter does not grow except in fresh water, either in very wet swamps or around the edges of fresh-water lakes, instances of which may be seen in the crater of Mount St. John and in the ancient crater now known as Lake Takapuna.
Resting upon this sandstone formation I found a deposit of two feet thick of a mud rock, similar in colour to the preceding stratum, but without any appearance of stratification, in which were many cylindrical holes lying more or less in a horizontal or inclined position.
These holes, of all diameters from half an inch to nine inches, clearly represent the branches and trunks of trees, as if a shower of mud and fine ashes had fallen from the volcano upon a young forest, or (which I think more probable) upon drift wood deposited at this and probably other points in the
fresh-water lake I am assuming to have occupied at this period the ancient crater. Almost every particle of the wood had decayed, except patches of bark adhering to portions of the cylindrical holes or matrices I have already described. So closely had the mud rock taken the impress of the trees, that I observed wherever a branch had been broken the most exact impress remained in the mud rock. Strata b, c, d represent the bottom of the ancient lake, at the lowest point of which will probably be found the stream which discharges into the Waitemata at Messrs. Low and Motion's mills, and known as the Western Springs. At some future date I may perhaps put down what miners call a “winze” on the dip of strata b, c, d, with the twofold view of obtaining more information regarding the waters of Mount Eden, and of bringing to light some of the plants and denizens of older times.
To return. Upon this mud rock there lay a stratum or lava stream of hard close-grained blue volcanic rock; and upon this again a stratum two feet thick of hard red scoria rock much honeycombed, followed by six inches of fine equal-sized scoria gravel.
These latter strata probably represent the first eruption of Mount Eden subsequent to the appearance of the more ancient crater above sea level, filling up the crater lake, and causing the superposed lava streams to dip at an angle of 25° from the mountain to N., or in the opposite direction to the dip of the lava stream below.
Lying on this I found a stratum (a) dipping at about 25° to N., one foot thick, of a reddish yellow clay, perforated with holes from 1/8 inch to 3/4 inch, perfectly round, smooth, and slightly taper; in some instances containing a woody fibrous-like matter, doubtless remains of the stringy filaments of fern root. On being touched these filaments fell away to a white ash. The fern roots growing on the surface exactly fitted these holes. All traces of the plants had disappeared, leaving only the impress of their roots in the baked clayey soil. Upon this lay a two-feet stratum of hard blue close-grained scoria rock; and upon this again a stratum of similar thickness of compact red scoria. It will not be necessary to continue to trace upward the various strata through which I passed, as they are fully described in the section.
In conclusion, I may say that during the exploration I had passed through five distinct eruptions before I had sunk to the depth of 176 feet. From the total absence of vegetable matter, or of any traces of decomposition of ash or rock, I conclude that these five eruptions followed each other at very short intervals. At 176 feet, as will have been seen, I came upon a deposit of volcanic mud, or possibly a decomposition of ash, showing a much longer interval of time to have occurred between this eruption and the first of the five subsequent ones. This eruption, occurring at a depth of about 180 feet from the present surface, probably filled up the lake occupying the crater of a
still more ancient volcano. This eruption, the first of the seven lava streams, undoubtedly occurred long prior to the second eruption, as shown by the three feet six inches of sandstone and mud in which the plants and forest were embedded. As to the period at which the latest eruption of the Mount Eden volcano occurred, we may conclude, from the twelve feet of brown soil found at the existing surface, that many centuries have elapsed since the last volcanic action
In ages long past there can be little doubt that the sea flowed across what is now the is now the isthmus on which Auckland on the east and Onehunga on the west stand, dividing what is now the North Island into two islands. Volcanic forces of tremendous power and long-continued action elevated the isthmus as we now see it, leaving the extinct volcanic cones of Mount Hobson, Mount St. John, One Tree Hill, and Mount Eden as mementos of the grand volcanic energy of former times.
Whether these extinct volcanos will yet again become active, time alone will tell.