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Da Farven er et karakteristisk Kjendemærke for den
ydre Adskillelse af de forskjellige Sedimenter, maa det være
af Interesse at kjende deres dermed sammenhængende
kemiske Egenskaber. I denne Anledning maa vi rette vor
Opmærksomhed mod den Oxydationsgrad, hvori Lerets
Jernmængder befinder sig.
For at faa saa mange Observationer som muligt i
denne Retning har jeg samtidig med Kalkbestemmelserne
tilberedt mig et saltsurt Udtræk (paa den forud omtalte
Maade) af den samme Prøve, hvis Kulsyregehalt var fundet
ved Johnsons Apparat. Jernoxydul er titreret med
Kamæleon og Jernoxyd med undersvovlsyrligt Natron og
Jodopløsning \
Som de senere Analyser vil vise, indeholder det
saltsure l’dtræk den væsentligste Del af Lerets samlede
Jernmængde.
Graat Ler.
Undersøgelsen af de talrige Bundprøver (over 150),
der er indsamlede paa Kystbankerne rundt det europæiske
Nordhav, har godtgjort, at den graa (stundom blaagraa.)
Farve i Almindelighed er en fælles Egenskab for alle de
Slamdannelser, der bedækker disse grundere Dele af
Havbunden. I åndre Retninger er imidlertid dette graa Ler
af en temmelig uligeartet Beskaffenhed.
Paa nogle Stationer finder vi det som en fin plastisk
Masse, der ved Tørring trækker sig stærkt sammen og
danner en haard Klump af en saadan Fasthed, at man
ofte maa bruge en Hammer for at slaa den istykker.
Andetsteds er Leret mere porøst og grovkornigt; det kryber
kun lidet sammen ved Tørring og bliver blot løst
sammenhængende.
Denne Uligeartethed er afhængig af Biblandingen af
grovere Materiale i Form af Sand (Kvartskorn) og
uorganiske Dyrelevninger.
Naar vi underkaster Leret en Slemning for at
undersøge dets Blandingsforhold, faar vi et større eller mindre
Residuum, der hovedsagelig indeholder Skaller eller
Skalbrudstykker af kalkafsondreiide Dyr og grovere mineralske
Partikler. De sidste bestaar som oftest for en væsentlig
Del af smaa Kvartskorn. der i Almindelighed er noget
afrundede i Kanterne. I hvilken høi Grad dette
uforgjænge-lige Mineral er fremherskende fremfor de øvrige i
Afleirin-gerne paa de grundere Dele af Havbunden, er paavist af
Delessé i hans Værk: -iLithologie du fond des mers"
En paalidelig kvantitativ Slemningsanalyse lader sig
vanskelig udføre i de tørrede Bundprøver. Ved Kogning
med Vand kan man vistnok faa den faste Lermasse opblødt,
1 Den sidste Methode’kan vistnok ikke gjøre Fordring paa nogen
stor Nøiagtighed, men maa dog ansees for tilfredsstillende ved det
foreliggende Øiemed.
Colour being the chief external feature distinguishing
oceanic deposits, it must obviously be of interest to learn
whence that character is derived: and on investigating the
subject, we find chemical agency to constitute the
originating cause, — viz. the more or less advanced stage of
oxidation in which the iron of the deposit occurs.
With a view to elucidate this question as fully as
possible, I prepared, when performing the carbonate of
lime determinations, a separate hydrochloric acid decoction
(in the manner previously described) of the same sample of
deposit in which the proportion of carbonic acid had been
found with Johnson’s apparatus. The protoxide of iron in
this decoction was titrated with permanganate of potash
and the sesquioxide with hyposulfite of soda and solution
of iodine.1
From the subsequent analyses it will appear that the
hydrochloric acid decoction contains the greater part of
iron present in the deposit.
Grey Clays.
A comparative examination of the numerous samples
of the bottom (upwards of 150) collected on the coastal
banks of the North-Atlantic, has shown that, as a rule, the
grey or bluish-grey colour constitutes a distinctive feature
common to all sedimentary deposits covering the shallower
portions of the sea-bed. In other respects, however, this
clay differs very considerably.
At some of the observing-stations it was found to
occur as a fine, plastic substance, which, when dried,
contracts into a lumpy mass, so hard and compact that a hammer
has frequently to be used for breaking it. In other
localities, this coastal clay is more porous in character and
coarsely granulated; it shrinks very little on exposure to
heat, and has, when dry, but trifling cohesive power.
This want of homogeneity arises from the presence
of coarser materials, such as sand (quartz) and inorganic
animal remains.
On thoroughly washing the clay, to determine the extent
of admixture, there will be found a greater or less residue,
composed chiefly of calcareous shells or fragments of such and
other mineral particles. Grains of quartz, as a rule rounded
at the edges, generally constitute the greater part of the
latter. The extent to which quartz predominates over other
mineral substances as a constituent of oceanic deposit in
the shallower parts of the sea-bed, has been shown by
De-lesse in his work "Lithologie du fond des mers."
To obtain a trustworthy determination of the various
mechanical constituents in the clay by washing, is hardly
practicable. The concrete substance of the deposit may. indeed,
1 Titration by this method does not indeed give a high degree
of accuracy, hut may for this purpose be regarded as yielding fairly
satisfactory results.
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