I'. Al 



iMIENS HANDLINGAR. BAND 29. 



tratclluric 



quartees 



•y therefon 



• regardi 



a surface i 



■ock of i 



oommonly voumled by måffnmtic resoiytion characteristic '.I' the 



dir effusive rocks. Fig. 17 shows an example. The simplest th( 



•■i sediment, thal has resulted from the mechanical destruction -I' a surface rock of th 



quartz-diorite faraily, is thal it is a. quartz-popkyrite- (dacite-)tuff. 



Unaltered porphyries or tuffs (the latter constituting the original Swedish halle^ 

 flinta»), do not occur in the Vestanå region, bu1 thej are, as well known, richly repre- 

 sented över all south-eastern Sweden and are found uot lar lo the north-east, in [Jrshull 

 and Ålmundsryd parishes, associated with contact-metamorphosed sediments of the Vestanå 

 dense fine-grained gneiss type, to judge by the materia] in the museum of the Geol. 

 Survey (collected in L872). 



The minerals occurring in the gneisses are andesine, orthöclase and microclinei 

 quartz, muscovite, biotite, hornblendé, epidote with allanite kernels, calcite, titanite, apa- 

 tite and zircon, iron oiv, pyrite and pyrrhotite. Of these, felspars, quartz, titanite, iron 

 ore, zircon and apatite always occur. The dense fine-grained gneisses also contain mus- 

 covite, epidote, calcite and, the least metamorphosed varieties excepted, greenish brown 

 biotite. When hornblendé appears, muscovite disappears. 1 Hornblendé is mostly found 

 in the gneisses, muscovite in the dense fine-grained gneisses, but there are exceptions 

 with hornblendé in the latter, muscovite (in large skeleton crystals) in the former, show- 

 ing, that the conträst between muscovite and hornblendé depends, not on the intensity 

 of metamorphism, but on differences in the chemical composition of the material. 



Structurally the contact-metamorphism manifests itself through developing in the 

 rocks a »hornfels»- or »honeycomb»-structure (Figs. 22, 17, 23, 26). The average sized grains 

 increase from 0,o-± to 0,?. — 0,5 mm. in diameter through the effect of the metamorphism. 

 Siniultaneously the crystallinity of all minerals increases; the leucoxene aggregates become 

 compact titanites, the small scales of muscovite become large skeleton crystals etc. The 

 quartz-felspar-aggregate is cleared by the disappea.rance of the minute particles of other 

 minerals. Other structural phenomena characteristic of contact-metamorphism are the 

 skeleton forms of the muscovites just mentioned (Fig. 33 and figures on p. 56), and the 

 poikilitic structure of the never idiomorphic hornblendé (Figs. 21, 27, 31). 



Especially charactei'istic of the gneisses of the district are the local accumulations 

 of the dark or the white minerals. Many of the less altered dense fine-grained gneisses 

 are mottled with dark and white spöts on a dark grey groundmass (Fig. 24). The dark 

 spöts are accumulations of biotite, hornblendé when present, epidote, titanite, iron ore, 

 and apatite. In the more metamorphosed varieties the minerals are larger and better 

 crystallized and epidote decreases. The Avhite spöts in the least altered, upper beds of the 

 dense fine-grained gneiss, probably derived from more weathered material, are composed 

 of epidote and muscovite with a back-ground of plagioclase in large grains. By increasing 

 metamorphism epidote and muscovite gradually disappear, and finally the accumulations 

 consist of pure, commonly untwinned andesine, seldom microcline, in polygonal grains, 



1 Of the more thau hundred slides of contact-metamorphosed gneisses from the district examined not a 

 single instance of muscovite and hornblendé occurring in the same slide was found. The two minerals exclude 

 one anöther, as well known, in eruptive rocks, this seems to be the case also in the contact-metamorphosed 

 types in opposition to the dynamo-mctamorphosed rocks. 



