GEOLOGY OF THE AREA. 9 



Gneiss. 



The gneiss of the area under examination is almost everywhere 

 very fine-grained and usually well-laminated. The chief minerals 

 found in it are quartz, felspar, hornblende, mica, garnets, magnetite, 

 and haematite ; and of these, quartz and hornblende are the most 

 abundant. 



According to the way in which these minerals are combined, the 

 gneiss may be divided into several varieties, of which the chief 

 are : — ■ 



(1) A very quartzose gneiss, with hornblende or mica. Gene- 



rally well-laminated. The hornblendic constituent is 

 variable in quantity and sometimes absent, so that we 

 find beds 1 of almost pure quartz. These, it may be 

 remarked, are not reefs, but beds. 



(2) Compact hornblendic gneiss. Occurs generally in bands, 



which behave as dykes, being quite independent of the 

 lamination of the surrounding gneiss. Nevertheless, 

 it is sometimes extremely difficult to make out a defi- 

 nite boundary. The rock seems to pass gradually into 

 the more quartzose and better-laminated gneiss at its 

 sides. It is probable that two sorts of gneiss have 

 been included under this head — the one being truly 

 eruptive, and the other forming part of the main 

 gneissic series. 



(3) Quartzo-hornblendic gneiss, with the crystals apparently 



arranged with their axes parallel to one another. This 

 is composed of small crystals and grains of quartz and 

 hornblende. It forms bands which stand up as con- 

 spicuous ridges, usually of no great length. The direc- 

 tion of these bands and of the lamination of the rock 

 is generally east to west. 



1 The term 'bed,' when applied to gneissic rocks in this paper, is used in its most 

 general sense ; and must not be understood to imply that these rocks are of sedimentary 

 origin. 



( 209 ) 



