12 G. F. Marris — Journey through Russia. 



before the more basic felspar in the phenocrysts, and the detailed 

 structure accompanying that phenomenon, form a very instructive 

 study. 



At the same time, the term rapakivi is largely applied by 

 the geologists of Finland to holocrystalline rocks of granitic type 



4^^MM^^ 



Fig. 1. — " Eapakivi granite " (^ nat. size). fl!=orthoclase ; 5=plagioclase. 



vphich contain large, rounded, and even irregularly - shajDed 

 phenocrysts of orthoclase, and which may not have the 

 triclinic felspar vsrapping round them, except in very rare 

 instances in any one massif. It is used, in fact, as a general 

 field term for granites of that description of Pre-Cambrian age. 

 As thus defined, "rapakivi granite" extends over enormous areas in 

 Southern Finland, on the north-eastern shore of Lake Ladoga, in 

 the little island of Hogland in the Gulf of Finland, and in the 

 Aland archipelago. It is one of the youngest of the Pre-Cambrian 

 eruptives in the country, and has been classified by Sederholm ^ in 

 his Jotnian formation — the younger subdivision of the Algonkian 

 (or Archseozoic) group. 



The " globular granites " exhibited at the offices of tbe Geological 

 Commission (some of which were subsequently shown in the 

 temporary museum formed in connection with the Congress meeting 

 at St. Petersburg) have been described in some detail by Benjamin 

 Frosterus.2 The accompanying diagram (Fig. 2) represents one of 

 these rounded or ovoid bodies. Mr. Frosterus tells me that the 

 kernel is frequently formed of a fragment of biotite schist. 

 Following his observations in the work just quoted, it will be seen 

 that, normally, there is a kernel or nucleus, outside of which are 

 four successive zones. In the diagram (Fig. 2) a represents the 

 nucleus, which, in the specimen analyzed by Frosterus, contained 

 63-64 per cent, plagioclase (Abg Anj) with 18-92 of biotite and 

 19-60 of quartz. Then followed the first coating or andesine zone 

 (b), in which the plagioclase (Ab4 Ku^ amounted to 73-22, biotite 

 8-00, and quartz 16-50. The next coating, the oligoclase-andesine 

 zone (c), yielded plagioclase (Abi An^) 56-07, biotite 5-60, and 

 quartz 3972. This is succeeded by a microcline zone (cZ), in which 

 we have plagioclase (Abg Aug) 16-52, alkali-felspar 5744, and 



' "Guide des Excursions," etc. (op. cit.), p. 18. 



'^ " Kugelgranit von Kangasniemi in Finland": Bull. Comni. Geol. Finlande, 

 No. 4, 1896. 



