PLATE XXXI. 



FIG. 1. 

 QUAKTZ-FELSPAR GRIT. 



LOCH MABEE, ROSS. 



Magnified 25 diameters. Ordinary light. 



The principal constituents are quartz (24) and felspar (10). 



The felspar in almost all cases retains its individual action under crossed 

 uicols and generally shows microperthitic or microlinic structures. The opaque 

 patches appear red by reflected light and are evidently local accumulations of 

 ferric oxide. The rock is of pre-Ordoviciau age. 



Fia. 2. 



QUARTZ-FELSITE. 



KIDLEES BURN, CHEVIOT DISTRICT. 



Magnified 50 diameters. Polarizer only. Short axis N. & S. 



The substances represented are quartz (24), biotite (9) and interstitial 

 matter. 



The quartz occurs in the corroded forms characteristic of the quartz- 

 porphyries. The interstitial matter is micro- or crypto-crystalline and contains 

 minute flecks and granules of ferrite. 



The rock forms a dyke in the Cheviot porphyrites. 



