176 JR. N. Lucas — The Older Rocks of Finland. 



That in spite of its foliation it is of eruptive origin is very well 

 shown by the fact that where it has gneiss for its next door neighbour, 

 the latter rock becomes impregnated with granite-magma, which, as 

 the gneiss-granite is the only granite near, cannot have been derived 

 from any other source. It is often said to pass into ordinary non- 

 foliated granite of a mineral composition identical with its own. I 

 feel very little doubt, however, that in many cases in which this is 

 stated to have been observed, and which I have not myself seen, the 

 facts are in reality similar to those in the instances I gave above, 

 namely, that the whole mass is one granite, foliated externally, and 

 losing its foliation inwards. 



Cases occur in which the gneiss-granite is pierced to such an extent 

 by bosses and veins of mc(ss««;e (unsheared) granite, doubtless of later 

 age, that it becomes a matter of diflficulty to decide which of the two 

 rocks in reality predominates. This state of things prevails in the 

 neighbourhood of the village of Numrais. 



Petrograpliically the gneiss-granite consists, like ordinary granite, 

 of quartz, felspar, and mica. The quartz and felspar are as a rule 

 disposed quite irregularly throughout the mass of the rock, but the 

 mica occurs in laminae which show parallelism of arrangement, giving 

 rise to a semi-schistose, at times fissile appearance. Thus the 

 apparent foliation of the rock is due entirely to the mica, and if we 

 could dissolve it out or eliminate it by any other means, we should 

 as a rule obtain a structureless mass of felspar and quartz. It is 

 consequently the mica which enables us to distinguish gneiss-granite 

 both from ordinar}?^ massive granite and from granite-gneiss. In the 

 former no order of arrangement whatever is observable among the 

 minerals composing it. In the latter there prevails a certain 

 parallelism of arrangement among all the minerals of which it con- 

 sists, and this would still remain to be observed even if any one of 

 them such as the mica were to be removed. In addition true gneiss 

 nearly always displays more or less stratification which is entirely 

 absent in the case of gneiss-granite, for the latter quite resembles 

 typical massive granite in its petrological uniformity and sameness 

 of colour over wide areas. 



Occasionally a certain subsidiary parallelism of mica cutting the 

 principal mica planes at an angle is observable. This appearance 

 we may, I think, correctly attribute to secondary stresses — it is in 

 fact the outward and visible sign of secondary cleavage. 



There is also a curious variety of gneiss-granite in which the role 

 ordinarily played by the mica is assumed by the quartz. In these 

 cases the quartz individuals are found as a species of laminae arranged 

 parallel to the general direction of cleavage and contributing to it 

 (Mantsila district). Where the quartz occurs in this manner, the 

 mica no longer shows parallelism — on the contrary, the mica and the 

 felspar are then disposed quite irregularly throughout the whole. 



RapaMvi, Sheet No. 7. — The towns of Borga and Friedrikshamn 

 are situated in the Gulf of Finland, about 70 miles apart. A square 

 erected upon a line joining the two would, roughly speaking, com- 

 prise the celebrated Eapakivi district of the South of Finland. The 



