R. iV". Lucas — The Older Rocks of Finland. 177 



■whole of this region is comparatively flat in character, consisting of 

 round dome-shaped protuberances covered with wood and alternating 

 with lakes and shallow valleys above the level of which they seldom 

 rise to a distance of more than about 50 or 100 feet. With the 

 exception of a few dykes and bosses of younger granite and syenite, 

 rapakivi forms the fundamental rock throughout the whole of this 

 district, and there is scarcely a hillock or elevation where it is not 

 to be seen emerging from beneath the Post-Tertiary formations which 

 frequently overlie it, and presenting to the view the large crumbling 

 boulders from which the rock has derived its name — rapakivi meaning 

 in Finnish rotten or crumbling stone. The aspect of these boulders 

 is very peculiar, at times picturesque ; on the weather-side they 

 have not unfrequently crumbled away to what is obviously not more 

 than half their original size, and the base of the rock is in conse- 

 quence covered with a talus of weathered fragments of felspar 

 quartz-grains, hornblende crystals, etc., among which the large egg- 

 shaped balls of orthoclase, varying from about an inch in diameter to 

 the size of the fist, are still to be found comparatively intact. 



Petrographically rapakivi consists of an aggregate of quartz, 

 orthoclase, biotite and hornblende — the last being always in evidence. 

 The orthoclase varies from pink to brick-red, occurring as a rule in 

 crystals of l^in. to 2^in. long and lin. to 2in. thick, with glassy 

 principal cleavage often appearing as Carlsbad-twins, and forming 

 the principal constituent of the rock. Owing to radial accretion, it 

 often happens that the crystals assume the form of oblong spheres 

 which occur notably in the neighbourhood of Elima. These large 

 crystals of orthoclase are surrounded by a mantle of oligoclase of a 

 yellowish-green colour turning white on weathering. The quartz 

 crystals, which are smoky grey or white, average about -lin. in 

 diameter. The mica, which is extremely black, is pretty equally 

 distributed throughout the whole in the form of brilliant laminae. 

 The hornblende, which is black, occasionally turning somewhat 

 green, occurs only in small quantities and in the form of short 

 monoclinic prisms. The rock does not display the slightest tendency 

 towards parallel arrangement of any of its constituents. 



Rapakivi has been microscopically examined by H. Gylling, from 

 whose report the following facts are taken. 



" In addition to orthoclase and oligoclase, microclinic felspar also 

 occurs, recognized by its peculiar structure and the fact that it 

 extinguishes polarized light 14° to 15° from the edge between the 

 clinopinacoid ^ and the basal plane. The microscopic examination 

 further shows that the quartz crystals are sometimes fully crystallized 

 out and contain water in minute cavities (fluid inclusions). Both 

 hornblende and oligoclase are much weathered, the former occa- 

 sionally being decomposed into calcite. As occasional constituents 

 occur yellow and yellowish-brown prisms of zircon, red laminse 

 and needles of heematite, and black opaque masses of magnetite 



_^ It will be seen that microcline is here regarded as really monoclinic — an opinion 

 ■with -which I certainly agree. 



DECADE III. VOL. A'lII. — NO. IV. 12 



