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disappearance of the porphyritic constituents and of the fiaser-structure. 

 It appears from the above description that the typical porphyroids (e.g., 

 those of the Ardennes) bear the same relation to quartz-porphyries that tho 

 " schistose greenstones " do to dolerites. They are for the most part acid 

 rocks that have been affected by dynamic metamorphism. 



Gneiss. This is a term used by the miners of Saxony to designate the 

 country rock in which the ore-deposits of the Erzgebirge occur. It has been 

 extended by geologists to rocks of very variable structure and composition. 

 Most gneisses resemble granite in composition. The term has, however, been 

 applied to rocks allied to syenite, diorite and gabbro. In all cases it 

 implies a more or less marked parallel structure in the arrangement of the 

 constituents. Some writers maintain that the term should be used merely 

 in a structural sense and that the different mineralogical varieties should 

 be designated by appending the name of the corresponding plutonic rock. 

 We should thus have granite -gneiss, diorite-gneiss, gabbro-gneiss, etc. Rocks 

 to which the term gneiss is now applied may be separated into three 

 groups: (1) Eruptive rocks in which the parallel structure dates from the 

 time of consolidation ; (2) rocks in which the parallel structure is ot 

 secondary origin and connected with the deformation of rock-masses after 

 consolidation ; and (3) rocks of doubtful origin. The third group is the 

 most important so far as distribution is concerned. It includes the 

 banded gneisses, so common in the Archaean areas, and supposed by some to 

 have been produced only in Archaean times. The rocks ot the first 

 group, so far as they are of acid composition, are of course merely varieties 

 of granite. The rocks of the second group are in most cases metamor- 

 phosed granites. In some cases they may be metamorphosed sediments of 

 granitic composition. (arkoses) or metamorphosed, possibly we ought to say 

 re-metamorphosed representatives of the third group. It thus appears that 

 only a limited portion of the rocks to which the term gneiss is applied can 

 be regarded as metamorphosed granites. 



Mica-schist. This term, like the last, is of very wide application and 

 includes rocks of diverse origin. It is applied to holo-crystalline foliated 

 rocks essentially composed of mica and quartz. The minerals of a typical 

 mica-schist are for the most part authigenic. The margins of granitic 

 masses in areas that have been affected by dynamic metamorphism, posterior 

 to the intrusion of the granite, often exhibit transitions from normal granite 

 through gneiss to mica-schist. The mica-schist in these cases must be 

 regarded as the result of the deformation of masses of granite. It is a 

 further stage of the process which has given rise to certain varieties of 

 gneiss. The mica has been developed at the expense of the felspar. Mica- 

 schists have also been produced by the metamorphosis of sedimentary 

 rocks and rocks of doubtful origin. Apart from field-evidence we have at 

 present no decisive tests for distinguishing between the different types of 

 mica-schist. 



Granulite. The so-called granulites of Saxony are regarded by 

 LEHMANN as eruptive rocks of granitic character that have been metamor- 

 phosed by plastic deformation subsequent to solidification. The term 



