27 Fra Hr. H. Beusch, Assistent ved den geologiske Unders0gelse, har jeg modtaget fplgende Meddelelse om bans mikroskopiske Unders0gelse af nogle Bergarter fra Jan Mayen. De Haandstykker fra Jan Mayen, som De velvilligen har tilstillet Universitetets Mineralkabinet, bar jeg efter Professor Kjerulfs Opfordring underspgt mikroskopisk. Der foreligger Basalter (Rosenbusch’s Definition). Herved er dog at bemserke, at Olivinen, idetmindste tildels, er tilstede i noget ringe Msengde, og at Plagioklasen, i Modssetning til, bvad der for det meste finder Sted bos de segte Basalter, for en Del forekommer porfyrisk indspraengt i storre Indi- vider. Alligevel har jeg ikke kunnet beslutte mig for Navnet Augitandesit. Fire af Haandstykkerne, No. 4, 5, 6, 7, var tennnelig ens; med blotte 0jne betragtet foreligger en temmelig rige- lig, af smaa tomme Blaererum opfvldt, ta3t, mprkegraa Bergart, hvori man ser fremskinne fine Feldspatlister og enkelte storre Feldspatkrystaller, saint ogsaa bennerker en eller anden Augitkrystal, undtagelsesvis endelig ogsaa et lidet Korn grpnlig Olivin. Under Mikroskopet ser man en forholdsvis lidet fin- kornet Grundmasse af langstrakte Plagioklaskrystaller og mere rundagtige Augitindivider, fremdeles mprkt slagge- agtigt Glas og Korn af en mork Jernerts. Udskilt i storre, porfyrisk indsprsengte Krystaller forekommer Plagioklas og Augit, bvilken sidste som Grundmassens er lys brunlig-gron, meget svagt pleocbi'oistisk. Hist og her i Praeparaterne opdager man indsprsengte storre Olivinkrystaller, der er saagodtsom aldeles friske og for en stor Del omgrsendsede af distincte Flader. I de storre Krystaller af de sidstnsevnte tre Mine- raler sees gjerne Glasindeslutninger og Jernertskorn, i Olivinen tillige Picotit (?). De som No. 2 og 3 mamkede Haandstykker var ikke porose og indeboldt talrigere samt mere fremtnedende por- fyrisk udskilte Krystaller end foregaaende. I Grundmassen var Augiterne meget smaa; Jernerts var rigelig tilstede; lidt Biotit bemerkedes; Glas saa man kun lidet til. Der- imod indeboldt de udskilte storre Plagioklaskrystaller Inde- slutninger af saadant som ogsaa af Grundmassen^ i vakkert rectangulsert omgraendsede Partier. En paafaldende Fin- kornetbed udmserkede den Olivinkrystallerne nsermest om- givende Del af Grundmassen, i bvilken forresten i dette lige saa lidt som i foregaaende Tilfaelde Olivin bemaerkedes som egentlig Bestanddel. Olivinen var dels omgraendset af Flader, dels bavde den ujevne Omrids, dels endebg traengte Grundmassen med ujevnt conturerede, undertiden udprae- get saekformede Forgreninger ind i dem. Disse Forgre- ninger var finkornede eller vel oftere et slaggeagtigt Glas, bvilket ogsaa gjerne optraadte i den til Krystallerne ellers allemmrmest stodende Del af Grundmassen. Hosstaaende tre Figurer, der er tegnede ved 360 Ganges Forstorrelse, • illustrerer nojere dette Forbold. For Tydeligbeds Skyld bar jeg undladt at indtegne de Spraekker og Jernertskorn. Mr. H. Beusch, Assistant to the Norwegian Geologi- cal Survey, has sent me the following results of bis micro- scopical examination of divers rock-specimens from the island of Jan Mayen. The rock-specimens from Jan Mayen which you kindly forwarded to the Mineralogical Museum of the University, I have, at Professor Kjerulfs request, submitted to micro- scopic examination. They are basalt (Rosenbuscli’s defini- tion). I must, however, observe, that in some cases olivine is present in no great proportion, and that plagioclase, as an exception to the general rule in true basalt, occurs here and there porphyrically imbedded in crystals of considerable size. Nevertheless, I cannot decide for augite-andesite. Four of the specimens, Nos. 4, 5, 6, and 7, are comparatively uniform in appearance. To the naked eye, their aspect is that of a dark-grey rock exhibiting numer- ous empty vesicles, together with glistening lines of feldspar and several large crystals of that substance; one or two crystals of augite may be likewise observed, and finally minute isolated granules of greenish olivine. Viewed under the microscope, is seen a comparatively coarse base of elongated plagioclase crystals, along with crystals of augite, rounder in form, dark slaggy glass, and grains of a dark-coloured iron ore. Plagioclase and augite occur imbedded in comparatively large crystals, the latter having, in common with that of the base, a brownish-green tint; it is, too, to a very slight extent pleocbroistic. Every here and there in the prepared specimens are observed comparatively large imbedded crystals of olivine, with scarcely a trace of decomposition, and having on all sides well- defined facets. In the three last-mentioned minerals are seen cavities containing glass and grains of iron ore; in the olivine also picotite (?). Specimens No. 2 and 3 are not porous; moreover, they differ from those described above in having a greater number of porphyrically imbedded ci’ystals, which are also more obvious. In the base, the grains of augite are ex- ceedingly minute; iron ore is present in great abundance; a little biotite, too, was observed, but only traces of glass. On the other band, the large plagioclase crystals exhibited numerous cavities containing the latter substance, as also that of the base, in beautifully formed rectangular spots. The part of the base immediately surrounding the crystals of olivine exhibits a remarkably fine granulation, though for the rest, neither in these nor any of the foregoing specimens does olivine occur as a true basic constituent. The crystals of olivine have some of them plane surfaces, others irregular outlines, and some are pierced by the sub- stance of the base with irregular, and possibly also sac- like, ramifications. These ramifications are either finely granular, or. more frequently perhaps, consist of slaggy glass, which often occurs too in the part of the base contiguous to the crystals. The three figures given below, showing the object as it appeared under the microscope 4*

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