27 



Fra Hi-. H. Beusch, Assistent ved den geologiske 

 Undersøgelse, har jeg modtaget følgende Meddelelse om 

 hans mikroskopiske Undersøgelse åf aogle Bergarter fra 



Jan Mayen. 



De Haandstykker fra Jan Mayen, som De velvilligen 

 har tilstillet Universitetets Mineralkahinet, bar jeg efter 

 Professor Kjerulfs Opfordring undersøgt mikroskopisk. Der 

 foreligger Basalter (Rosenbuschs Derinition). Herved er 

 dog at bemærke, at Olivinen, idetmindste tildels, er tilstede 

 i noget ringe Mængde, og at Plagioklasen, i Modsætning 

 til, livad der for det meste linder Sted bos de ægte Basalter. 

 for en Del forekommer porfyrisk indsprængt i større Indi- 

 vider. Alligevel har jeg ikke kunnet beslutte mig for 

 Navnet Augitandesit. 



Fire af Haandstykkerne. No. 4. 5. 6, 7. var temmelig 

 ens; med blotte Øjne betragtet foreligger en temmelig rige- 

 lig, af smaa tomme Blærerum opfyldt, tæt, morkegraa Bergart, 

 hvori man ser fremskinne rine Feldspatlister og enkelte 

 større Feldspatkrystaller. samt ogsaa bemærker en eller 

 anden Augitkrystal. undtagelsesvis endelig ogsaa et lidet 

 Korn grønlig Olivin. 



Inder Mikroskopet ser man en forholdsvis lidet fin- 

 kornet Grundmasse af langstrakte Plagioklaskrystaller og 

 mere rundagtige Augitindivider. fremdeles mørkt slagge- 

 autigt Glas og Korn af en mork Jemerts. Udskilt i større, 

 porfyrisk indsprængte Krystaller forekommer Plagioklas og 

 Augit, hvilken sidste som Grundmassens er lys brunlig-gron. 

 meget svagt pleochroistisk. Hist og her i Præparaterne 

 opdager man indsprængte større Olivinkrystaller, der er 

 saagodtsom aldeles friske og for en stor Del omgrændsede 

 af distincte Flader. 



I de større Krystaller af de sidstnævnte tre Mine- 

 raler sees gjerne Glasindeslutninger og Jernertskorn. i 

 Olivinen tillige Picotit (?). 



De som No. 2 og 3 mærkede Haandstykker var ikke 

 porøse og indeholdt talrigere samt mere fremtrædende por- 

 fyrisk adskilte Krystaller end foregaaende. I Grundmassen 

 var Augiterne meget smaa; Jernerts var rigelig tilstede; 

 lidt Biotit bemerkedes; Glas saa man kun lidet til. Der- 

 imod indeholdt de tidskilte større Plagioklaskrystaller Inde- 

 slutuinger af saadant som ogsaa af Grundmassen i vakkert 

 rectangulært omgrændsede Partier. En paafaldende Fin- 

 kornethed udmærkede den Olivinkrystallerne nærmest om- 

 givende Del af Grundmassen. i hvilken forresten i dette 

 lige saa lidt som i foregaaende Tilfælde Olivin bemærkedes 

 som egentlig Bestanddel. Olivinen var dels omgrændset af 

 Flader. dels havde den ujevne Omrids, dels endebg trængte 

 Grundmassen med ujevnt conturerede. undertiden udpræ- 

 get sæklbrmede Forgreninger ind i dem. Disse Forgre- 

 ninger var finkornede eller vel oftere et slaggeagtigt Glas. 

 hvilket ogsaa gjerne optraadte i den til Krystallerne ellers 

 allernærmest stedende Del af Grundmassen. Hosstaaende 

 tre Figurer, der er tegnede ved 360 Ganges Forstørrelse, 

 illustrerer nojere dette Forhold. For Tydeligheds Skyld 

 bar jeg undladt at indtegne de Sprækker og Jernertskorn. 



Mr. H Beusch, Assistant to the Norwegian Geologi- 

 cal Survey, has sent me the tollowing results of his micro- 

 scopical examination of divers rock-specimens from the 

 island of dan 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- 

 scopie examination. They are basalt (Rosenbuschs defini- 

 tionj. I must. however. observe. fchat in some cases olivine 

 is present in no great proportion, and that plagioclase. as 

 an exception to the general rule in true basalt, occurs bere. 

 and there porphyrically imbedded in crystals of considerable 

 size. Nevertheless. I cannot decide for .augite-andesite. 



Four of the specimens. Nos. 4, 5, ti. 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 Hnally 

 minute isolated granules of greenisb olivine. 



Yiewed under the microscope. is seen a coinparatively 

 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 pleochroistic. Every 

 bere 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 crystals. which are also 

 more obvious. In the base. the grains of augite are ex- 

 ceedingly minute; iron ore is present in great abuudance; 

 a little biotite, too, was observed, but only traces of glass. 

 On the other hand. 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 foregoiug 

 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* 



