M. N. Lucas — Geology of Finland. 295 



ancient member of the Archaean series. "Wherever it is exposed 

 to view it appears as emerging from beneath younger Laurentian 

 strata which have been deposited directly upon it, or it is covered 

 as far as can be seen conformably by the younger micaceous 

 gneiss into which it passes by insensible degrees. Thus in 

 the neighbourhood of Paajarvi it is overlain for a considerable 

 distance by hornblende gneiss and eurite, from which it is sharply 

 separated. As a rule it is much pierced by intrusive granite of 

 probably similar age, which has found its way in between the layers 

 of the granite-gneiss, and which, having taken on a parallelism of 

 structure through the influence of lateral pressure, is often on super- 

 ficial examination hardly to be distinguished from it, and has then 

 received the name of gneiss-granite. The mineralogical composition 

 of granite-gneiss is as a rule extremely uniform, consisting in vai'y- 

 ing proportions of light grey and grey fatty-looking orthoclase, and 

 white pearly oligoclase, very finely striated grey to white transparent 

 and semi-transparent quartz and black mica. At times a certain 

 amount of red orthoclase mingles with these constituents, causing 

 the rock to assume a reddish colour, and in the neighbourhood of 

 the above-mentioned granite masses it passes over completely into 

 the red oligoclase gneiss-granite. 



Microscopic sections of the granite-gneiss of Southern Finland, 

 kindly placed by Prof. Wiik at my disposal during my 6tay in 

 Helsingfors, were found to display with marked distinctness the 

 peculiarities of granite-gneiss that distinguish it both from ordinary 

 granite and from gneiss-granite. The crystals of quartz and felspar, 

 particularly of the former, are seldom completely developed, but 

 pi'esent a rounded appearance, suggestive of crystalline aggregation 

 about originally attrited grains, in this respect resembling strongly 

 the appearance under the microscope of the more crystalline Devonian 

 . " grauwackes." The crystals of mica are usually well developed, 

 and do not in any way present peculiarities similar to those of the 

 felspar and quartz crystals. They occasionally give evidence of 

 internal movements within the rock, being broken across, and one or 

 more of the fragments being transported a short distance in a direc- 

 tion parallel to the general banding ; but there are no long streams 

 of mica microliths, as is observed in sheared granites, and the quartz 

 and felspar crystals fill in without interruption the space between 

 the mica fragments. This would seem to point to the movements in 

 question having originated during a pasty or semi-pasty condition 

 of the rock, rather than to effects of pressure or shearing stress in 

 a rock perfectly hard and cr3 7 stalline at the time. 



Grey Gneiss. — The grey micaceous gneiss, which forms the next 

 higher member of the series, overlies the older granite-gneiss into 

 which it often gradually passes, as in the neighbourhood of Borgnas. 

 It has the same general strike as the gneiss-granite, that is to say, 

 roughly speaking, E. and W., though both at times deviate a good 

 deal from this general rule. In its normal condition it consists of 

 black mica, generally white felspar and quartz. It is frequently 

 much cut up and pierced by veins of granite, as for instance in the 



