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29
Disse mikroskopiske Forhold minder unægtelig om
Eruptiver, som bliver finkornede paa sine Grænser og
sender finkornede Forgreninger ud i de omgivende
Bergarter. Hvorvidt her foreligger et Afkjølingsfænomen er
dog tvivlsomt; maaske Olivinen allerede ved sin
Udkrystallisation har paavirket Grundmassen, saa den i dens
umiddelbare Nærhed har stivnet hurtigere. Lignende Forhold
som disse ved Olivinen beskrevne iagttages, 0111 end mindre
karakteristisk, ved de udskilte Plagioklas- og Augitkrystaller.
Haandstykket No. 8 er porøst, af en forholdsvis lys,
rødliggraa Farve og indeholder udskilte •Augitkrystaller.
Under Mikroskop ser man, at Bergarten, som er forholdsvis
lidet grovkornet og lidet rig paa mindre Augitindivider,
indeholder en hel Del Olivin. Dennes Individer udhæver sig
ikke synderlig i Størrelse fremfor de øvrige Bestanddele;
den er ikke som i de foregaaende Tilfælde frisk, men
underkastet en begyndende Serpentinisering ledsaget af
Udskillelse af Jernoxyd, som er det, der gjør Bergarten rødlig.
Slaggeagtigt Glas er temmelig rigelig tilstede. Nogle lange
fine Naale i Feldspaten formodes at være Apatit.
Haandstykket No. 1 udmærker sig fra de beskrevne
makroskopisk derved at det ingen porfyrisk udskilte
Feld-spatkrystaller indeholder, men derimod en Mængde smukke
gulagtiggrønne Olivinkrystaller, som er indtil 0.5 cm. store.
Under Mikroskopet ser man, at Hulrummene gjerne
er omgivne med en Zone af Glas. I Olivinen, der synes
at være udkrystalliseret efter en anden Typus encl
foregaaende. bemærkes hyppig det som Picotit tydede Mineral. Den
mørke Jernerts er tilstede ikke alene i rundagtige Korn,
men ogsaa i stavformede Legemer. Disse forekommer som
Regel nær Olivinkrystallerne og har en bestemt Stilling til
disse, uanseet Bergartens øvrige paa kryds og tvers
liggende Bestanddele. De staar, saavidt jeg har kunnet
iagttage det, lodret mod Olivinernes Hovedakse, parallelt deres
lange Biakse; de staar nemlig lodret mod deres bedste
Gjen-nemgangsretning. Man faar ludtrykket af, at Bergartens
først udskilte Bestanddele, Olivinen og Jernertseu, i den
endnu plastiske Masse har ordnet sig i et bestemt Forhold
indbyrdes.
These microscopic details are undeniably suggestive
of eruptive rocks that exhibit a fine granulation at their
limits and send forth finely granulated ramifications. Whether
we have here the result of some cooling process is doubtful;
possibly, the olivine acted in the course of its crystallisation
upon the basic substance, thereby causing the latter in its
immediate vicinity to harden sooner. A similar feature,
though less characteristic than in the olivine, distinguishes
the imbedded crystals of plagioclase and augite.
Specimen No. 8 is porous, of a lightish
ruddy-grey colour, and contains imbedded crystals of augite.
Viewed under the microscope, this rock, which,
comparatively, has not a coarse granulation and exhibits but few
particles of augite. contains a good deal of olivine. The
prystals of the latter substance are not very large as
compared with its other constituents: the olivine is not as in
the foregoing specimens undecomposed. but exhibits distinct
traces of serpentinisation, along with -the formation of oxide
of iron. This it is which gives a red colour to the rock.
Slaggy glass occurs in comparative abundance. A few long
thin crystals in the feldspar would appear to be apatite.
Specimen !No. 1 is distinguished macroscopically from
those described above, by its not containing imbedded
crystals of feldspar; it exhibits, however, an abundance of
beautiful yellowish-green crystals of olivine, measuring up to
0.5 cm.
Viewed under the microscope, the hollow cavities are
found to be encompassed by a zone of glass. The olivine
exhibits a type of crystallisation different to that observed
in the other specimens, and contains a greater proportion
of picotite than usual. The dark iron ore occurs not only
in roundish grains, but also in rod-shaped corpuscles. These
corpuscles are as a rule observed in immediate proximity to
the crystals of olivine and have a definite position towards
them, irrespective of the other constituents of the rock,
that run in all directions. They are placed, so far as I
could determine, perpendicular to the vertical axis of the
crystals, and parallel to the macrodiagonal. being perpendicular
to the most perfect cleavage. The first-formed
constituents of the rock, olivine and iron ore, would appear to
have taken up a definite position one towards the other
while the surrounding mass was vet plastic.
Olivinkrystal i Basalt. — A Crystal of Olivine in Basalt.
Den i stavformede Legemer forekommende Jernerts liar en
bestemt Stilling i Forhold til Olivinkrystallen. Tegnet ved 360
Ganges Forstørrelse.
The iron ore occurring as rod-shaped corpuscles has a definite
position towards the crystal of olivine. — Microscope magnifying 3(50
diameters.
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