E. JSf. Lucas— The Older Rochs of Finland. 173 



moderately fine-grained cystalline aggregate, in which little patches 

 of black hornblende are seen moulding the dull whitish felspars. 



In a slice [1258] the felspar is seen to be a lamellated plagioclase 

 in crystal-plates, idiomorphic towards the bisilicates, but interfering 

 with one another. Some crj^stals have their zones of growth indicated 

 by slight differences in optical properties. The wide extinction- 

 angles point to a basic composition. The hornblende has a pale 

 green-brown colour, with a fibrous structure. It is sometimes a 

 mass of matted fibres, though more usually there is a commou 

 orientation throughout each plate. There is no doubt that the 

 mineral is pseudomorphic after augite, and there is frequently au 

 unaltered kernel of colourless augite in the centre of the uralitic 

 aggregate. In places, however, the usual amphibole cleavage is seen, 

 and this is accompanied by more pronounced colour and pleochroism. 

 The only other prominent mineral is magnetite, in irregular patches, 

 of later formation than the felspar, but moulded by the bisilicate. 

 The structure of the rock points to a plutonic origin, the general 

 characters being those of a gabbro rather than a diabase, though to 

 some extent transitional. 



rpj 



V. — Notes on the Older Egoks of Finland. 



By R. N. Lucas, B.A. 



{Continued from page 299, Vol. VII. July, 1890.) 



?HE consideration of the Archeean rocks of Finland, as pointed 

 I out in a previous article,' naturally divides itself into two 

 parts : I. The treatment of the stratified or foliated members of 

 the series ; II. That of the igneous or eruptive rocks. In describing 

 the former, want of space compelled me to handle the subject in 

 what I fear must be regarded as a very fragmentary manner. In 

 addition I confined myself entirely to giving an account of the 

 older members of the formation, omitting all mention of the younger 

 schists, mica-clay slates, phyllites, etc., which go to make up what 

 is in Finland regarded as corresponding to the Huronian subdivision. 

 I did this from two reasons, — firstlj^ because I was personally better 

 acquainted with the rocks in question ; secondly, because it seemed 

 to me that it is the problem of the origin, sequence and composition 

 of the older members of the series which at the present day pos- 

 sesses most interest for the student of the geology of the Archaean, 

 and in consequence is the one with regard to which the greatest 

 divergence of opinion prevails. In the present article I propose 

 to give a short account of the more important eruptive rocks of 

 Finland ; but before proceeding to do so, it would not, I think, be 

 out of place to furnish a short resume of the conclusions to which 

 the study of the foliated Archsean rocks of the country has led 

 me, and which I believe to be thoroughly supported by the data 

 published in my former article. These are : — 



1. The succession on which I especially insist, namely, commencing 

 from the base — (1) Granite-gneiss; (2) Grey micaceous gneiss; 

 1 Geol. Mag. July, 1890. 



