R. N. Lucas — Geology of Finland. 297 



occurrence of magnesia, mica, and microline (in specimens from 

 Horis and Sukkola). 



Hornblende- Gneiss and Eurite. — These rocks, which together form 

 the principal member of the Upper Laurentian series, occur almost 

 always together, alternating with and passing into one another to 

 such an extent as to necessitate their being considered in conjunction. 

 They thus form one stratified system, and occur as in the neighbour- 

 hood of Koskis and Hattina, on either side of belts of granite-gneiss, 

 which they overlie, and presumably in some places cover ; their 

 strike being almost always parallel to that of the latter formation. 

 All the members of this series, whether containing hornblende 

 or not, present a more or less fine-grained appearance, and the 

 bands of micaceous gneiss which are at times interbedded approach 

 very closely to the structure of eurite, and might appropriately 

 be termed eurite-gneiss. Mineralogically the composition of this 

 latter rock is the same as that of ordinary gneiss, white ortho- 

 clase and oligoclase in small, weathered grains with black mica 

 in small shining lamina?, and small white transparent grains of 

 quartz. At times the grain is so close that the rock becomes a true 

 eurite, the separate minerals composing which are only visible 

 under the microscope. It always contains some hornblende, and 

 passes, as the amount of that mineral increases, into a perfect horn- 

 blende-gneiss or hornblende-eurite, as the case may be. Iron and 

 arsenical pyrites occur in considerable quantities as accidental con- 

 stituents. In certain places, however, the hornblende-gneiss appears 

 more independently and less interstratified with the rocks here 

 described. Such is the case to the south of Helsingfors, at Boe, 

 and in Lill Perno Bay near Borga. Here it often contains a quantity 

 of calcite grains, which by weathering out cause it to acquire a very 

 riddled appearance, and it is much intersected and broken through 

 by granite veins and dykes. 



A specimen of the rock from Boe was examined microscopically 

 byCastren, who says : "The rock principally consists of hornblende 

 and felspar, the latter generally being triclinia The hornblende 

 forms single grains, which are well shaped and occur as dark and 

 light varieties, and the rock sometimes contains small quantities of 

 a mineral of the pyroxene group. A section of a small crystal of 

 this mineral that, having grown into a hornblende crystal, had 

 fragmentary crystallographic surfaces, showed distinct and less 

 distinct pinakoidal cleavage, and extinguished polarized light in a 

 direction parallel to the plane of cleavage, the plane of the optic 

 axes being normal to the plane of most perfect cleavage. Another 

 section, in which the cleavage planes were parallel, showed an 

 extinction angle of o0°. This points to the crystal being diallage. 

 Prismatic cleavage was not perceptible in the fii'st section, and in 

 it the prism was more developed than the pinakoid. Apatite and 

 derived epidote grains occur as accidental constituents." 



Another specimen from Stigsbole was found by Castren to consist 

 of triclinic felspar and hornblende both of the dark and light green 

 variety, together with quartz and a certain amount of magnesia mica. 



