Yol. 65.] PROF. J. H. L. VOGI ON LABKADOEITE-NOEITE. 83 



quantity of secondary hornblende ; magnetite (titanomagnetite) ; 

 a little biotite ; an extremely small quantity of spinel, apatite, and 

 pyrite ; a very little garnet, as a border round the iron-ore, rarely 

 round the ferromagnesian silicates ; also alteration-products, which 

 are present, however, only in very small quantity. Olivine was 

 not observed in any of the slides. 



The porphyritic labradorite. — This mineral is thick- 

 tabular along M(010) and bounded by M (010),_P (001), 1 (110), 

 T(110), and also as far as could be seen by x (101), perhaps too by 

 y (201). It occurs in Carlsbad twins, with occasional albite- 

 twinning and subordinate pericline-twinning. Its delimitation 

 from the ground-mass is sharp. Between the very large pheno- 

 crysts occur smaller ones of exactly similar character. On the cut 

 and polished surface, of which part is shown in fig. 1 (p. 82), are 

 seen phenocrysts of all sizes, ranging from about 60 square centi- 

 metres down to 1 or 2. The distance between the centres of the 

 large phenocrysts on the surface illustrated in fig. 1 varies mostly 

 from 8 to 18 centimetres. 



As t o the different composition of the plagioclase 

 in the phenocrysts and in the ground-mass, it is seen 

 macroscopically that the porphyritic plagioclase is violet-brown, a 

 colour such as is produced by the ordinary microlithic interposi- 

 tions, while the plagioclase of the ground-mass is light grey with 

 only a slight tinge of brown. A separation of the ground-mass- 

 minerals, first with a horseshoe-magnet, then with an electro- 

 magnet, and lastly with Sonstadt's solution, was carried out by my 

 son Mr. Th. Vogt, mineralogical student. 



(1) The specific gravity was determined by means of the 

 Westphal balance, in combination with the heavy liquid, and 

 yielded the following results :■ — 



(a) Porphyritic plagioclase; the bulk was of sp. gr. = 2-708, and some 

 lighter particles, occurring in very small quantity, were only a few 

 thousandths lower. 



(b) Ground-mass-plagioclase ; sp. gi\=2-685. There was no question 

 as to these differences in specific gravity having been correctly determined. 



(2) The chemical composition. — Analysis No. 1 (average of 

 four analyses carried out by students in the Christiania University 

 Metallurgical Laboratory under my direction) is of the phenocrysts. 

 The material for these analyses was a little impure, to the extent 

 of 1 or 2 per cent., owing to the presence of small quantities of 

 magnetite, biotite, and pyroxene. 



Analysis No. 2 was carried out by Mr. Th. Vogt on isolated 

 ground-mass-plagioclase (sp. gr. =2-685), of the same large block 

 (illustrated in fig. 1) as Analysis No. 1. 



For comparison, especially with the porphyritic plagioclase, I 

 also append an analysis, carried out in my own laboratory, of labra- 

 dorite (with a small admixture of iron-ore and ferromagnesian 

 silicates) from the labradorite-rock at Andopen on Plakstado, 

 3| kilometres south of the porphyritic labradorite-norite. 



g2 



