Vol. 65.] PEOF. J. H. L. VOGT OK LABEADORITE-NOEITE. 81 



5. On Labeadobite - Noeite with Poephybitic Labeadorite - 

 Ceystals : a Contribution to the Study of the ' Gabbeoidal 

 Eutectic.' By Prof. Johan H. L. Yogt, F.H.G.S. (Read 

 January 13th, 1909.) 



The object of this paper is to contribute to the investigation, on a 

 physico-chemical basis, of the sequence of crystallization in the 

 gabbroidal eruptives. 



The Lofoten and Vesteraal Islands on the northern coast of 

 Norway (lat. 67° 50' to 69° N.) constitute, as is well known, a petro- 

 graphical province with a very considerable number of eruptives, 

 among which orthoclase-plagioclase-rocks, especially monzonite and 

 banatite, are the most frequent. 



On one of the islands, Plakstado, there is an area of labradorite- 

 rock l comparatively poor in ferromagnesian silicates and mag- 

 netite, besides labradorite-norite with somewhat larger quantities 

 of the same. Under the name ' labradorite-norite ' is denoted 

 a rock which lies chemically and mineralogically on the boundary 

 between labradorite-rock and norite. 



The field-exposure of the labradorite-rock with the labradorite- 

 norite has a length of 164 kilometres, a breadth averaging from 2| 

 to 3| kilometres, and an area of about 50 square kilometres. To this 

 must be added its continuation under the sea. At the northern 

 margin, towards the Archaean in the neighbourhood of Napp Farm 

 on Plakstado, the labradorite-norite occurs with a porphyritic 

 development, that is, with phenocrysts of labradorite-felspar, 

 often of remarkably large dimensions. We will now examine this 

 latter rock more closely. 



The porphyritic labradorite-norite of Napp, like most 

 of the rocks belonging to the eruptive province in question, generally 

 shows little or no sign of pressure. It is in most places coarsely 

 crystalline, with labradorite-phenocrysts measuring as much as 15 

 or 18 centimetres in length and from 6 to 8 in breadth. It has a 

 hypidiomorphic ground-mass of relatively coarse grain. Occasion- 

 ally, however, the rock is more finely-grained, both as regards 

 ground-mass and labradorite-phenocrysts, which are here only 

 from 2 to 3 centimetres long. 



An example of the ordinary coarse-grained rock is shown in 

 fig. 1 (p. 82), which illustrates part of the cut and polished surface 

 of a large block. 2 The minerals which occur therein are plagioclase 

 (labradorite) : namely, on the one hand, the plagioclase-phenocrysts, 

 and on the other, the plagioclase (somewhat richer in albite) of 

 the ground-mass; hypersthene and diallage, with quite a small 



1 In it basic segi-egations are found, on the one hand of titanic iron-ores 

 (titanomagnetite-diallagite and titanomagnetite-spinellite), and on the other 

 hand of peridotites, occasionally also amphibolite : see J. H. L. Vogt, Zeitschr. 

 f. prakt. Geol. vol. viii (1900) p. 233. 



2 I was unable to obtain any usefully illustrative photograph from the 

 surface, and so I have preferred to make a freehand drawing. 



Q. J. G. S. No. 258. g 



