ti 



characterised by the prominence of its pink pegmatite 

 bands ; a fourth, by the number of its dark basic dykes ; 

 a fifth :iconsuuit beds and lenticles of limestone 



or graphitic schist 



But it has seldom been found possible to determine 

 any satisfactory order of succession among these rocks, 

 or to be sure that what seems to be such an order has 

 really any chronological significance, The observer will 

 sooo discover proofs of intense disturbance among these 

 masses. He will find them crushed, fractured, crumpled, 

 and pushed over each other by thrust-planes, and he may 

 be driven to conclude that the divisional planes, though 

 here and there they may simulate bedding, may be 

 entirely due to some other cause. He will not infre- 

 quently be led at last to regard a whole series of gneisses 

 as originally a complex mass of eruptive rocks of different 

 ages which have been injected through each other, and 

 have subsequently been crushed and sheared In such 

 circumstances, probably all that he can hope to establish 

 will be the order of appearance of the various materials 

 It may be possible to discover, from the evidence of 

 intrusive veins, which portions of the series are newest 

 Sometimes, where no such veins are available, the dose- 

 grained or " chilled " edge, which betokens the contact of 

 an injected mass with a cooler and older rock, may have 

 in some measure survived the general metarnorphism, 

 and be still traceable (see p. 161). Much assistance in 

 an investigation of this kind may be derived from a study 

 of the structure of a Paleozoic or even Tertiary core of 

 eruptive material, consisting of a complex series of 

 intrusions. 



