R. N. Lucas — The Older Rocks of Finland. 175 



Lanrentian age. In the neighbourhood of Joensuu, however, a 

 rock coiTesponding in all respects to the rapakivi of the south and 

 west, was discovered by Prof. Wiik contorting and dislocating 

 Huronian slates and phyllites, and hence it has become customary 

 to consider the rapakivi as a "younger" eruptive rock. The case 

 of the diabases is very similar. No grounds could be advanced 

 which would lead to the conclusion that the dykes and bosses of 

 the north of Ladoga Lake (Wallamo) are much more recent than 

 the Huronian period. But in the extreme west of Finland a strip 

 of sandstone occurs which is usually regarded as Cambrian, and this 

 sandstone is pierced by diabase dykes. There is a strong family 

 resemblance between nearly all the Finnish diabases, and it would 

 certainly not be a very violent hypothesis to look upon them all as 

 approximately of the same age, i.e. Cambrian or later— how much 

 later we cannot form even an approximate opinion, as in Finland 

 there is a " Gronungagap" from early Cambrian to Quaternary times. 

 I now pass to the consideration in detail of the more important 

 igneous rocks following the natural subdivisions of the subject, and 

 commencing with a description of — 



1. The Acid Eruptive Eocks. 



Gneiss- Granite. — The base of the crystalline series we found to 

 consist of the granite-gneiss — a rock in which coarseness of banding 

 and indistinctness of foliation cause it to present a superficial 

 resemblance to granite. In the gneiss-granite we have a granite 

 which, through the influence of pressure, shearing-stress, or other 

 causes, has assumed the appearance of a fissile gneiss, and which 

 might be very appropriately termed a foliated granite. In many 

 instances it appears to be the oldest member of the eruptive series ; 

 for in most cases where it is pierced by unaltered granite, the latter 

 is plainly a younger rock. I think, however, it would be incorrect 

 to assume that all gneiss-granites are of the same age, for I have 

 seen plenty of evidence to make me incline to the opinion that 

 gneiss-granite is in reality a fades assumed by granites which may 

 differ in age. Among other reasons for thinking this to be the 

 case, I may mention that I have most distinctly observed at least 

 one case in which a boss of granite which was mineralogically 

 homogeneous throughout, showed unmistakeable foliation near its 

 edges, which became less distinct and finally disappeared on 

 approaching the centre of the mass. This, I think, can only be 

 explained by assuming that pressure and shearing-stress have made 

 their influence more felt upon the exterior portions than upon the 

 interior. 



Statigraphically the gneiss-granite usually preserves relations of 

 more or less intimacy either with the granite-gneiss or with the 

 grey gneiss, running side by side with one or other of these rocks 

 over considerable tracts of country. Its foliation always appears 

 parallel to the strike of the gneisses — a fact which certainly favoiirs 

 the view that it was the same pressure which tilted the gneiss strata 

 into their vertical position which produced the foliation of the granite. 



