of Igneous Rocks. 195 



gneiss is found, in which all the minerals are surrounded by reaction 

 rims of alkali felspar; around the biotite this alkali felspar occurs in 

 small grains without definite orientation, but around the old felspar 

 (partly kaolinized orthoclase, oligoclase, and perthite) the new water- 

 clear felspar has consolidated in optical continuity. Owing to extensive 

 interaction between the gneiss and the nepheline-syenite, there is 

 a gradual transition from the one to the other through forms in which 

 the original orthoclase, oligoclase, and perthite of the gneiss have 

 been almost completely changed to newly formed microperthite of 

 orthoclase and albite. Q/uartz meanwhile takes a subordinate position, 

 and segirine-augite is developed along with sphene and biotite, the 

 latter showing the habit prevalent in the intrusive mass. It is difficult 

 to say where the altered gneiss ends and the acid border facies of the 

 intrusion begins. For convenience Hogbom has drawn the outer 

 limit of the nepheline-syenite at the place where quartz first appears 

 in the rock. 



2. The Plutonic Rocks. 



The two chief types are nepheline-syenite and an almost pure 

 calcite rock or limestone. There is every gradation between these 

 two, depending on the proportion of the constituent minerals. Certain 

 important basic segregations also occur. There is no regular 

 distribution of the different types, and even within one hand-specimen 

 sevei'al varieties may be found. No definite order of crystallization 

 among the minerals is to be observed. Sometimes the femic minerals 

 crystallize before the salic, but often the reverse is the case. Calcite 

 as a rule comes last of all. 



The chief minerals are — 



Pink orthoclase, which contains a noteworthy amount of barium. 



Pink nepheline, sometimes seen in beautiful idiomorphic crystals, 

 standing out on weathered surfaces owing to the solution of enclosing 

 calcite. It is occasionally completely altered into acicular zeolites. 



Fresh cancrinite. Its primary character is clearly shown by its 

 mode of occurrence in large plates, into which well-defined crystals of 

 felspar and nepheline project, and in which idiomorphic sphene and 

 pyroxene are scattered ; in this mode of occurrence it obviously has 

 not been derived from nepheline. It is also found filling up cracks, 

 and is generally the last mineral to crystallize except calcite. The 

 composition of cancrinite corresponds so closely with the assumption 

 that it is a hydrated compound of nepheline and calcite that its 

 presence in these rocks is just what might be expected. Sometimes 

 it is bordered on one side by nepheline, and on the other by calcite, 

 but at the same time it is rare or absent in the limestones even where 

 nepheline is abundant. 



Calcite, very prevalent in almost all varieties of these rocks. It is 

 generally the last mineral to crystallize, and is found in allotriomorphic 

 grains. But it also occurs in av ell-formed crystals, for Hogbom (1) 

 has described a fine-grained nepheline-syenite with porphyritic calcite 

 crystals, partly contaminated by syenite minerals. 



Pyroxene, either very slightly pleochroic segirine-augite or more 

 pleochroic segirine. 



