120 Backström, vestanåfältet. 



The amphibolite of this bed between the quartzite and the gneisses is mineralogi- 

 cally characterised by hornblende and sometimes biotite and about as much plagioclase, 

 with subordinate orthoclase and quartz. Epidote often occurs — sometimes in nodules — 

 whilst iron ore, titanite, and apatite were observed in some varieties, though in other 

 they were absent. '■ Chlorite and calcite were not observed. Structurally there are a num- 

 ber of varieties. The felspar always occurs as small polygonal grains, and in some cases 

 hornblende and mica are also quite formless and there is therefore produced a »hornfels» 

 structure (Fig. 13). In other cases the hornblende occurs as prisras spread in a ground- 

 mass of felspar with small crystals of epidote, titanite, mica etc. (Fig. 14). A third vari- 

 ety is reproduced in Fig. 15, nere the hornblende occurs as large prismatic grains, not 

 idiomörphic but with irregular borders. — The chemical composition of one of these atn- 

 phibolites from east-north-east of Boana was found to be: 



SiOa AI2O3 Fe 2 Os FeO MgO CaO NaaO K2O H2O Sum. 



50,30 14,07 7,05 5,31 7,18 8,06 4,02 2,26 1,61 99,86. 



This being the composition of a cliabase (though somewhat rich in alkalies) it only 

 remains to show, that amphibolites of the kinds just described may be produced from 

 diabases through metamorphism, and that the agencies required to produce it have been 

 at work in the Vestanå region. 



The examples which have been described of diabases changed into amphibolites by 

 purely dynamical metamorphism are rare. Of these the products studied by Lössen, 1 

 Milch 2 and G. H. Williams 8 are ver}' different from the Vestanå amphibolites both struc- 

 turally and nrineralogically, chlorite, sericite, and calcite being commou constituents of 

 thein. An example of »the metamorphism of dolerite into hornblende-schist» described by 

 Teall 4 on the other hand shows a result identical with the amphibolites in question. 

 Still this until now appears to be an exception, and we have no right to ascribe the 

 metamorphism of the supposed cliabase to the comparatively slight orographic moveinents 

 manifested in this part of the region, especially as we find rocks with about the same 

 chemical composition as the amphibolites, but rnore resembling the common dynamometa- 

 morphic diabase type, in the north-western parts of the district, where the accompan_ying 

 granites and gneisses show the effects of great pressure. 



A summary of the results attained by the study of the alterations of diabases through 

 eontact-mdamorphism shows the Vestanå amphibolites closely to agree with intensely 

 metamorphosed diabases and diabase-tuffs. As both the »Hälen» and »Jemshög»-granites 

 of the Vestanå district are younger than the gneiss, that conformably underlies the great 

 bed of amphibolite, it is probable, that they are also younger than the amphibolite, and 

 consequently, the amphibolite having the chemical, mineralogical and structural proper ties 

 of diabase modified by contact-metamorphism and being within the contact-zone of younger 



1 Jalirb. d. preuss. geol. Landesanstalt 1883, p. 618 and 1884, p. 525 and several other papers. 



2 Zeitscbr. d. d. geol. Ges. 41. p. 394 (1889). 



3 Bull. U. S. Geol. Survey N:o 62 (1890). 



4 Quart. Journ. Geol. Soc. 41. p. 133 (1885) and >.Britisli Petrögraptiy»; p. 154 and 197, plate 20 and 21. 



