THE PRE- CAMBRIAN ROCKS, BRITISH ISLES. 5 



of Scotland, the large area of the Outer Hebrides, which con- 

 sists of similar gneisses, remaining to be explored. It is 

 therefore possible that indisputable evidence of an ancient sedi- 

 mentary series through which the gneiss was originally protruded, 

 may yet be discovered in the unexplored islands. But taking 

 the gneiss as at present known in Sutherland and Rosshire, 

 we find it to be generally coarse in texture, rudely foliated, and 

 passing sometimes into massive types in which foliation is either 

 faintly developed or entirely absent. Much of this gneiss is 

 considerably more basic than the more t3'pical rocks to 

 which the term gneiss was formerly restricted. It consists of 

 plagioclase felspar with pyroxene, hornblende, and magnetite, 

 sometimes with blue opalescent quartz, and sometimes with black 

 mica. These predominant minerals are segregated in different 

 proportions in the different bands, some bands consisting mainly 

 of pyroxene or hornblende, with little or no plagioclase, others 

 chiefly of plagioclase, with small quantities of the ferro-magne- 

 sian minerals and quartz, others of plagioclase and quartz, others 

 of magnetite. This separation of mineral constituents can hardly 

 be attributed to mere mechanical deformation. It rather resem- 

 bles the segregation layers which may be studied in intrusive sills 

 and other deep-seated masses of eruptive material, and which 

 are obviously due to a process of separation that went on while 

 the igneous magma was still in a liquid or viscous condition. 

 At the same time it is manifest that extensive dynamical changes 

 have affected the rocks since the appearance of this original 

 banded structure. 



There is further evidence that beside the original eruptive 

 masses, which for want of any means of discriminating their rela- 

 tive dates of protrusion must in the meantime be regarded as 

 belonging to one eruptive period, other portions of igneous 

 material have been subsequently and at successive epochs, after 

 the first mechanical deformations, injected into the body of the 

 original gneiss. These consist of dykes of basalt and dolerite, 

 followed by still more basic peridotites and picrites, and lastly by 

 emanations from a distinctly acid magma in the form of granites. 



