ELA0LITE-SYEN1TE. 197 



The biotitc which occurs in this rock is brown in colour, and 

 presents a peculiar corroded appearance due to ramifications of one 

 of the white minerals, producing a structure not unlike the " quartz 

 of corrosion n in many gneisses and granulites. The hornblende 

 is olive-green to brown-green in colour according to the direction 

 of the section, with a very strong pleochroism : a = brownish yellow ; 

 b = deep olive-green, almost black on account of the intense absorp- 

 tion, and c = as dark as b. Some sections consequently remain 

 almost black in all positions of the Nicols. The extinction-angle 

 (c : r) is about 12 . The felspar^ which forms the main mass of 

 the rock, is almost wholly microperthite, and occurs in granular 

 crystals devoid of idiomorphic outlines. 



The principal feature of interest is connected with the pre- 

 sence of calcite in granular crystals, with apparently as much right 

 as any of the others to be considered a primary constituent. The 

 crystals form isolated granules, and there are no signs of secondary 

 decomposition, or structures which suggest its infiltration into the 

 rock. This is not the first time that calcite has been found as a 

 constituent of elaeolite-syenite and been regarded as primary in 

 origin, and the low silica percentage in this group of rocks removes 

 the chief theoretical difficulty to its crystallization from a molten magma 

 as a normal constituent of an igneous rock. In the Alno occurrence, 

 well known from the description by.Tornebohm, the more recent 

 researches of Hogbom tend to show that the large masses of 

 crystalline limestone, as well as the scattered granules of calcite, 

 have been fused in the magma without decomposition, and that 

 during the process of solidification calcite has crystallized out of the 

 magma in precisely the same way as the other minerals. 1 



1 Hogbom. " Ueber das Nephelinsyenitgebiet auf der Insel Alno." G.F.i 

 Stockholm Fijrhdl., Vol. XVII (1895), pp. 100 and 214. Abstract in Mitt. Mag., 

 Vol. XI (1897), p. 250, and Rosenbusch, Mxkr, Fhys. (1896), pp. 169 and 171. 



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