185 



quent and perfect, but is effected with great faci- 

 lity by the loss, either of the mica, or of the fel- 

 spar, or of both. 



Of a gradation between gneiss and primary 

 sandstone, the island of Sky affords a very per- 

 fect example: and it is a highly interesting in- 

 stance, as the latter is, in a great degree, a rock of 

 a decidedly mechanical structure. The process 

 by which the change is effected, is the gradual 

 substitution, first of chlorite schist, and ultimately 

 of mica, for the argillaceous schist which alter- 

 nates with the layers of sandstone ; which, at the 

 same time, become gradually thinner, more in- 

 definite, and more perfectly cystalline. The two 

 rocks are, in other places, often associated in fre- 

 quent alternation. 



The gradation above-mentioned, includes 

 that of gneiss into chlorite schist; which also 

 occurs in other situations, producing interme- 

 diate varieties that are enumerated in their proper 

 places. 



A gradation between gneiss and argillaceous 



schist, is not uncommon in Scotland; and, as 



.. might be expected, this change takes place where 



