72 



GNEISS. 



slate ; — gneiss has often a waved form. This rock has been repre- 

 sented as stratified, I conceive, by a mistake, in confounding the strat- 

 ified with the slaty structure : the latter is occasioned by the quantity 

 of mica, and sometimes of talc which it contains, and is the effect 

 of crystallization.^ 



Beds of crystalline limestone, and of hornblende rock, occur in 

 gneiss. It contains most of the metallic ores, both in veins and beds. 

 Crystals or garnets are frequently interspersed in gneiss, but are more 

 common in micaceous schist, which is nearly allied to this rock. 



In many parts of the world, the declivities of granite mountains 

 are covered by rocks of gneiss. Gneiss constitutes the principal rock 

 formation in a considerable part of Sweden. It occurs in Scotland 

 and Ireland, but is scarcely known in any part of England or Wales. 

 Very well characterised gneiss occurs in the vicinity of Aberdeen. 

 An imperfectly formed gneiss is found on the Malvern Hills. 1 have 

 also seen gneiss, brought from the lower part of Skiddaw in Cum- 

 berland. Mountains of gneiss, are not so steep and broken as those 

 of granite, and the summits are generally rounded. 



Mica-sla(e, or Micaceous Schistus, is frequently incumbent on 

 gneiss, or granite, and covered by common slate : it passes, by gra- 

 dation, into both these rocks — the coarser grained resembling gneiss, 

 and the finer kind, by insensible transition, becoming clay slate. 



Mica slate is composed essentially, of mica and quartz, intimately 

 combined ; the felspar, which is a principal constituent part of gran- 

 ite and gneiss, occurs, only occasionally, in irregular masses in this 

 rock. The colour of mica slate is, generally, a silvery or pearly 

 white, inclining to a bluish grey or a light green ; it is sometimes 

 nearly black, and when weathered, is generally yellow. I have a 

 specimen of mica slate from North America, which has the purple 

 colour of the amethyst ; but such deviations from the common col- 

 ours are rare. 



Crystals of garnet are frequently disseminated in mica-slate : it 

 contains, occasionally, crystals of other minerals. It has a slaty 

 structure, and is, often waved and contorted, and divided by thin 

 laminae of quartz. It, sometimes, contains beds and laminae of crys- 

 talline limestone, or is intermixed with serpentine. Frequently also 

 mica slate, contains beds and veins of metallic ores. The gradation 

 of mica slate into gneiss and clay slate, and the transition from gran- 

 ite to mica-slate, may be seen distinctly in some of the rocks near 

 Bray, in the county of Wicklow in Ireland, where I observed that 

 the beds of mica-slate adjoining the granite, are traversed by nume- 



* The partings or divisions in rocks, which may be properly denominated rents 

 are distinct from those which are the effect of crystallization, and may be distin- 

 guished by their irregularity and roughness, and the indeterminate manner in 

 which they intersect the stone. Some partings have evidently been the result of 

 mechanical causes. 



