i8o OUTLINES oi I 1 1 1 .1 - -KOLOGY PARTI 



. l>ands of a different composition, such as folia of 

 hornblende and quartz, or of felspar and mica, or of 

 some other combination of the same or other minerals 

 sufficient to mark off these bands from the gneiss. 

 Hence the gneiss itself is a thick bed or bed-like 



Fie. 70. Crumpfing and thrust-planes in gneiss south coast of Mull. 



and its continuation must be traced along, and not across, 

 the line of foliation. 



Among Archaean rocks no such regularity of outcrops 

 is met with as we find among stratified formations. But 

 broad bands of rock, differing from each other in mineral 

 characters, may be recognised and traced. Sometimes 

 we encounter a zone of grey gneiss with perhaps little or 

 no mica; then may come another zone abounding in 

 black mica or in hornblende. A third tract may U 



