296 R. N. Lucas — Geology of Finland. 



neighbourhood of Vibtjarvi, where it is so much impregnated with 

 calcite as to effervesce when treated with acid. Not far from Valkjarvi 

 Lake it is bordered by hornblende schists, when it becomes very 

 rich in hornblende, and often contains magnetic iron ore. At times it 

 passes into a fine-grained, dark grey rock resembling a felsitic schist, 

 and occasionally it becomes very micaceous and contains chlorite. 

 Granite veins and dykes often penetrate it to a very great extent, 

 the granite running through the gneiss, the gneiss interbedded with 

 and surrounded by granite ; and sometimes large fragments of gneiss 

 have been broken off and enveloped in the eruptive rock, as in the 

 district around Borgo and near Mansikiri, to such a degree that it 

 becomes almost impossible to say whether the rock should be 

 described as granite or gneiss. 



A red variety of micaceous gneiss occurs at a few spots, as for 

 instance near Morskom, where it strikes E. to W., and is very fine- 

 grained, becoming at times euritic, and consists of reddish-brown 

 orthoclase, yellowish-red plagioclase, grey and white quartz and 

 green mica. 



Both varieties of micaceous gneiss are at times very rich in 

 garnet, which mineral tends to replace the mica, and they then 

 become red and white garnet-gneiss, as at Nurmijarvi and Koskis, 

 which at times (as near Loji) growing finer in structure passes into 

 garnet-mica-schist. 



A graphite-gneiss has also been observed cutting in a narrow 

 band through a hill of granite near Kukilu church. 



In contact with granite the gneiss presents a series of phenomena 

 interesting in their irregularity. In some cases it runs along the 

 surface of contact without any indications of being affected by the 

 contiguity of the eruptive rock. At other times, though there is 

 nothing of the nature of transition from one rock to the other, and 

 the line of separation is sharply defined, the gneiss in proximity to 

 the granite becomes much distorted, and then generally appears of 

 finer grain. Often large portions are broken off and imbedded in 

 the granite, and the foliation of these detached portions is then much 

 contorted. This occurs notably in Sibbo Bay on the south coast. A 

 peculiar facies, which is doubtless a contact phenomenon, occurs 

 occasionally in which the felspar individuals are much larger and 

 coarser, and the whole mass becomes porphj^ritic. 



These varying phenomena may in all probability be best explained 

 by the supposition that the gneiss was in some instances deposited 

 directly upon solidified granite, in others it has been subjected to the 

 modifying effects, both mechanical, physical, and chemical, of a 

 lengthened contact with masses of granite in a state of fusion. 

 That this latter state of things prevailed over extensive areas and 

 for a considerable length of time is rendered probable, not only by 

 the phenomena of disruption and contortion caused by the intrusive 

 action of the granite alluded to above, but also by the gradual 

 passage of grey gneiss into gneiss-granite not unfrequently observed, 

 as at Nackskog. 



Microscopic examination has occasionally brought to light the 



