178 R. H. Lucas — The Older RocTis of Finland. 



and titaniferous iron ore. Crystals of fluor-spar are also at times 

 visible even to the naked eye." 



Though as above pointed out the rapakivi is almost perfectly 

 uniform in its characteristics throughout the district in which it 

 occurs, a varietj'^ distinguished by comparative hardness and capacity 

 for resisting the weather is also found, as a rule on the confines of 

 the rapakivi region proper ; for instance, in Morskom, Perno, and 

 Borga parish, where, as also in Helsingfors, it is largely used as a 

 building-stone. In this variety there is usually less oligoclase and 

 hornblende, and the orthoclase and quartz crystals are of larger size. 



The remarkable tendency to fall to pieces under the action of the 

 weather, which, if we except the occurrence of microcline, is perhaps 

 the most characteristic peculiarity of rapakivi, has given rise to a 

 variety of attempted explanations. Some have inclined to the 

 opinion that the phenomenon in question is due to the presence of 

 iron. A strongly weathered rapakivi from Lapptrask, however, 

 was found to contain 762 per cent, of iron, while an unweathered 

 sample from Elima gave as much as 6'53 per cent. — a difference in 

 amount hardly sufficient to explain the observed variations in 

 behaviour towards the weather. Others again attribute the opposite 

 effects observed in the two varieties to the different proportions of 

 silica. Here are some analyses bearing upon the question : — 



Silica. 



A non-weathered rapakivi from Stromfors 67*44 



A similar rapakivi from Elima 69-17 



Rapakivi from Korsmalm, near Lapptrask, much weathered 77"61 



There seems, however, to be no reason why a high percentage of 

 silica should conduce to weathering, and it does not always appear 

 to be the case that the weathered specimens contain more silica than 

 the unweathered. Probably Moberg is right in attributing the 

 principal role in the matter to the oligoclase which weathers easily 

 and occurs in greater amount in the weathered samples. 



At a few places within the rapakivi territory it is pierced by 

 dykes of a younger granite consisting mainly of orthoclase and 

 quartz as at Norrby, near Lapptrask. 



Eapakivi is everywhere very much jointed and the jointing takes 

 place in certain regular directions. 



This peculiar rock is, as is well known, confined exclusively to 

 Finland, with the exception of the boulders which were transported 

 thence to the North German Plains by the agency of ice during the 

 Glacial Period, where they are to be met with very frequently and 

 have been elaborately described by v. Ungern Sternberg (N. Jahrb. f. 

 Min. 1882). 



There is another rapakivi district in the West of Finland, in 

 the neighbourhood, namely, of Nystad. Here the rock differs a 

 good deal from the southern and more characteristic type described 

 above. The structure there so common, according to which the 

 orthoclase balls are surrounded by a sort of mantle of oligoclase, 

 is here seldom observed. On the contrary, the orthoclase usually 

 occurs in separate crystals of considerable dimensions, and the 



