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50
Ler. som ikke er noget almindeligt Forekomststed for hine
Foraminiferer, eller som altsaa ligger imellem Kystleret og
det saakaldte Biloculinler, har jeg paa Kartet afgrændset
for sig selv under Navn af Overgangsier.
Det forholder sig imidlertid ikke saaledes, at det graa
Ler ophører, hvor det brune begynder at forekomme.
Tvertimod finder vi ofte, at det førstnævnte vedbliver at udgjøre
det underste Lag af Bundprøven selv indenfor det egentlige
Biloculinlers Omraade. Det viser sig saaledes, at det brune
Sediment kun meget langsomt tiltager i Mægtighed. Paa
fiere af Expeditionen Tversnit fandtes det graa Ler som
en væsentlig Del af Bundprøven indtil en Afstand af ca.
15 Mile fra den første Station, hvor det brune Ler havde
begyndt at vise sig. Et Par af de Bundprøver, der var
optagne paa meget store Dyb (over 1700 Favne), bestod
udelukkende af graat Ler og indeholdt kun ganske smaa
Mængder af det brune. Disse Observationer tyder paa. at
Biloculinleret er en Dannelse af ringe Mægtiglied, hvilende
paa et underliggende Lag af graat Ler. Dette stadfæstes
end yderligere ved Skrabningerne. under hvilke en Mængde
graat Ler optoges paa de største Dybder.
I Overgangsleret forekommer Biloculiner og de øvrige
Dybvands-Foraminiferer blot spredt og enkeltvis. Skjønt
det derfor i Almindelighed kun bruser svagt for Syrer,
hænder det dog stundom, at Kalkgehalten er temmelig
betydelig, selv om ingen Skaller kau iagttages hverken i det
oprindelige Ler eller i dets Residuum efter Slemning.
Indenfor de Grændser, hvor de ovennævnte
Foraminiferer optræder med Regelmæssighed og i større Antal, er
det brune Ler meget fint og ensartet og synes selv under
en stærk Lupe kun at bestaa af amorfe Partikler.
Biloculinlerets brune Farve kan være mere eller
mindre udpræget. Nogle Bundprøver er lyse gulbrune, andre
mørkbrune, i fugtig Tilstand næsten chokoladefar vede.
Talrige smaa, hvide Foraminiferer bidrager naturligvis til at
give Leret en lysere Farve.
Ved Slemning af Biloculinleret erholder man et større
eller mindre Residuum, der væsentlig bestaar af
Foraminiferer af forskjellig Størrelse, Fonn og Farve. Under Lupen
viser det sig dog, at nogle yderst smaa, runde Kalkskaller
med et tydeligt Kammersystem er langt overveiende i Antal
fremfor de øvrige. Professor G. O. Sars har været af den
Godhed at bestemme disse Dyr for mig. Han har fundet,
at de hører til de meget bekjendte "Globigeriner", men at
de er meget mindre udviklede med Hensyn til Størrelse
end de, der forekommer i de sydligere Have.
Da disse Dyr overalt i Nordhavet er Biloculinernes
uadskillelige Ledsagere, kunde maaske det her omhandlede
Sediment ogsaa gaa ind under Navnet -’Globigerinler", en
Betegnelse, der er blevet benyttet i de britiske Expeditionen
Rapporter. Ifølge John Murray skal dette Sediment, hvis
mest typiske Eiendommelighed er dets Rigdom paa
Globi-geriner, forekomme i stor Udstrækning i det af
Challenger-expeditionen beseilede Hav, hvor det er fundet paa de fleste
of the brown clay. The strip of brown clay, varying in
width, where that deposit does not regularly contain
Biloculinæ, I have marked ofl’ on the map as a separate
formation, under the name of transition clay.
Meanwhile, it is by no means to be supposed that
the grey clay suddenly terminates where the brown begins
.to occur. On the contrary, the former deposit was
frequently found to constitute the under layer of samples brought
up from within the limits of the true biloculina formation.
Vertically, therefore, the increase of the brown deposit is
very slow. In several of the localities of the tract explored,
the grey clay was found to constitute a large part of every
sample, within a distance of 15 geographical miles from
the first Station at which the brown clay occurred. One
or two samples, obtained from great depths (upwards of
1700 fathoms), consisted almost exclusively of grey’ clay,
the admixture of brown deposit being very small. From
these data, Biloculina clay is shown to be a deposit of
trifling vertical extent, with an underlying layer of grey
clay. Moreover, a good deal of grey clay came up in the
dredge from the greatest depths.
In the transition clay, Bilondince and the other
deep-sea species of Foraminifera occur but occasionally, and
scattered. Hence, though as a rule, therefore, that deposit
effervesces very slightly with acids, the proportion of lime
is sometimes considerable, even when no calcareous shells
can be detected either in the clay itself or the residue left
on washing it.
When regularly characterised by the presence of the
above-mentioned Foraminifera in considerable numbers, the
brown clay is an exceedingly fine and homogeneous deposit,
and would appear, even if examined under a powerful
magnifier, to consist of amorphous matter.
The brown colour of Biloculina clay varies
considerably in depth and distinctness. Some of the samples were
light yellowish-brown, others dark-brown, — nay almost of
a chocolate shade in a damp state. Numbers of minute
white Foraminifera naturally contribute to give the clay a
whiter tint.
On washing Biloculina clay, there is a greater or less
residue, which chiefly consists of divers species of
Foraminifera, varying in magnitude, form, and colour. With
the aid of a lens, minute, round, calcareous shells having
a distinct system of chambers are found to exceed by far
in number the other animal remains. Professor G. O. Sars
has had the kindness to determine these animals. He
found them to be known Globigerinæ, though much less
developed in point of size than the species which occur in
the Southern Seas.
These animals invariably accompanying Biloculinæ in
the North Atlantic, the brown deposit here treated of might
be also termed ,lGlobigerina clay," a designation adopted
in the Reports from the various British Expeditions.
According to Mr. John Murray, this sedimentary substance,
whose chief typical characteristic consists in the presence
of great numbers of Globigerinæ, occurs extensively
throughout the tracts of ocean investigated on the Challenger"
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