164 The American Naturalist [February, 



certain areas became colloidal, then rapid crystallization was set up 



and the branching forms resulted. Though the main features of 



the Rapakiwi granite have long been well-known through the descrip- 

 tions of Ungern-Sternberg, but little information has been granted 

 us as to its occurrence and structural peculiarities. A recent article 

 by Sederholm 4 gives an account of the varieties of the rock and out- 

 lines their modes of occurrence. The peculiarity common to all varie- 

 ties is the occurrence of porphyritic crystals and the possession of a 

 granophyric ground mass. The prevailing type possesses phenocrysts 

 having an elliptical form and surrounded by a rim of oligoclase. The 

 orthoclase is never pure, but it contains plagioclase particles and grains 

 of quartz, and biotite or hornblende, the usual constituents of the 

 ground mass. These inclusions are often arranged concentrically. 

 The peculiarity of the structure of the ground mass is the idiomor- 

 phism of the quartz, which is often intergrown with the feldspar, lepid- 

 omelane and hornblende in micropegmatitic forms. The place of the 

 orthoclase phenocrysts is sometimes taken by an aggregate of orthoclase 

 and quartz grains, surrounded by a radiating rim of orthoclase and an 

 exterior one of plagioclase. Miarolitic cavities are filled with fluorite. 

 As the orthoclase becomes smaller the structure of the rock becomes 

 more granitic, at the same time the amount of orthoclase decreases 

 and mi crocline takes its place. The finest -iain.«l varieties occur as 

 dykes in the others, and are finely granophyric. All these varieties 

 occur in the Wiborg district in South Finland, where, on account of 

 their remarkably easy weathering and the consequent production 

 of granitic debris, they are well known. This easy weathering is 

 ascribed by the author to mechanical rather than chemical agencies. 

 At Aland and other regions types are found resembling more or less 

 closely those described. In some porphyritic crystals of oligoclase 

 occur in a micropegmatitic ground mass of quartz and orthoclase, and 

 in others porphyritic quartzes in a granophyric ground mass. Between 

 the branches of the quartz in the granophyre are small areas of coarse 

 grain, and in these are found the miarolitic cavities. Not only do the 

 rocks described occur in Southern Finland, but they are found also in 

 the Southwestern portion of the same country, as well as on the islands 

 off its coast and in the Eastern part of Sweden. All the varieties are 

 supposed to be phases of the same ma-ma. tin- coar,o-graiued, deep- 

 seated facies and the granophyric surface forms. For granitic rocks 

 with idiomorphic quartz the author proposes to use the descriptive 

 *Min. n. Petrog. Mitlh., xu, p. 1. 



