128 NERBUDDA DISTRICT. 



These trap dykes have visibly exercised considerable influence on the 



Alteration of rock by texture and structure of the rocks they traverse, 

 Trap dykes palpably altering them at and near the planes of 



contact. This subsequent alteration by the contact of trap rocks has 

 necessarily rendered it difficult to assign the exact amount of change 

 which is due to the previous contact with the granite rocks, although 

 there is no possibility of doubting after careful examination of the phe- 

 nomena, that to this earlier contact with molten matter a large propor- 

 tion of this alteration is due. 



This attempt to establish a connection of cause and effect between the 



„ A , granite intrusion and the alteration of the schiat 

 Metamorphisin ot the D 



schists, rocks in these localities considered in detail 



naturally suggests the wider question of the relation of these rocks over 

 the whole area occupied by both jointly: and although we have no 

 theory, or even hypothesis, to offer on the subject of metamorphic action 

 generally, and least of all would wish to support that theory which 

 would attribute the present state of the mineralization of the vast thick- 

 ness of crystalline schists of the Nerbudda Valley, to the presence 

 among them of granitic rocks, still it is nevertheless interesting to trace 

 such relations between these rocks as may now be observable at the 

 surface. In attempting this it will be convenient to examine these 

 relations first as chemical or mineralogical, then as mechanical. 



When speaking of the boundaries of the granite masses it was 

 _ , .. „ , stated that, as a rule, they are very indistinct, 



Relations of granite and ' ^ ■* 



schist rocks; mineral. t ] ie bedded and the igneous rocks passing into 

 each other by insensible gradations — the cases of distinct granite dykes 

 above described being exceptional ; indeed although those cases are in 

 finely exposed sections, and quite satisfactory in themselves, the granite 

 being typically the granite of the country, and the schist equally clearly 

 belonging to the great metamorphic series, still they are isolated facts, and 

 similar instances have been but seldom observed elsewhere. 



