Gneiss. 43 



such as occur in the unaltered rock, and the chemical 

 action (brought into activity by heat and moisture) 

 which induced their development, may perhaps in some 

 cases have been assisted by sublimations proceeding 

 from melted matter below. The intensity in many 

 countries of these metamorphisms, extending over many 

 thousands of square miles (as in Scotland, Norway and 

 Sweden, and Canada), and through rocks thousands of feet 

 in thickness, proves that it was the result of a long-con- 

 tinued process, taking place probably in all cases at 

 considerable depths. The whole has then been up- 

 heaved and disturbed, often many times, and after de- 

 nudation the gneissic and the more thoroughly meta- 

 morphosed and sometimes intrusive granitic rocks were 

 at length exposed at the surface. 



Some of the metamorphic rocks, which I have to 

 explain, have been highly disturbed, and in the north 

 occupy about one-half of Scotland. Most of this area 

 includes, and lies north-west of, the Grampian moun- 

 tains ; and I must endeavour to explain by what 

 processes metamorphism of rocks has taken place, not 

 in detail, but simply in such a mariner as to give a general 

 idea of the subject. 



I have already said that typical gneiss consists of 

 irregular laminae of mica, quartz, and felspar, and it 

 frequently happens that they are bent, or rather 

 minutely folded, in a great number of convolutions, so 

 small, that in a few yards of gneiss they may sometimes 

 be counted by the hundred. All these metamorphic 

 rocks and granite, were by the old geologists called 

 Primary or Primitive strata, and were considered to 

 have been formed in the earliest stages of the world's 

 history, because in those countries that were first 

 geologically described, they were supposed to lie always 



