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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 25, 



fied * ; that the concentric ferruginous zones to which they gave 

 birth determined the structure of the ultimate plutonic rock, and, to 

 a certain extent, the arrangement of hornblendic and other ferrugi- 

 nous minerals ; and were perhaps the channels by which an electro- 

 chemical operation, emanating immediately from the walls, was made 

 to pervade all the adjacent parts of the rocks, until crystallization 

 was induced, and the metamorphism was complete ? 



The district, considered as more or less metamorphosed, exhibits 

 two sets of phsenomena, one where the ferruginous walls and lines are 

 the principal feature, and the other where they are rare or absent. 

 In the first case the plutonic action appears to have extended to great 

 distances from its place of full operation, and in fact to have had no 

 limit but the length of fissures and planes of discontinuity in the 

 superincumbent rocks. On the other hand it has often been extremely 

 weak, hardly extending beyond the walls, and leaving the adjoining 

 rock unaltered. In the second case the plutonic action appears to 

 have from the first pervaded the whole mass of rock, as far as it ope- 

 rated, and to have produced the following succession of changes, 

 examples of all which occur : — 1st. The original rock is indurated in 

 different degrees ; 2nd, a sub crystalline texture is induced, but with- 

 out any perfect crystals being generated ; 3rd, crystals of particular 

 minerals begin to appear in this basis ; 4th, the same process is con- 

 tinued and various forms of crystalline rock are produced, the granitic 

 being that where the conditions of crystallization were most favour- 

 able, either from the longer continuance or greater intensity of the 

 plutonic action. The process when preceded by the formation of 

 ferruginous walls is the same in effect, as these merely tend to deter- 

 mine the manner in which the iron is diffused, the lines where cry- 

 stallization begins, and particular minerals are generated, the struc- 

 ture in the mass of the regenerated rock, and the distance to which 

 the plutonic action, in its various degrees and with its various results, 

 extends. 



It is evident that fusion did not precede crystallization in the pro- 

 gress of change up to the formation of the highly crystalline por- 

 phyries of Pulo Nanas, South Point, and many places in the eastern 

 islands, and in the greenstones and other highly hornblendic rocks 

 which are undoubtedly the common dark shale of the District dis- 

 guised, since the graduation from the one to the other can be traced. 

 On the other hand there is no direct evidence that the granitic form 

 was preceded by fusion, while all the facts we have been considering, 

 particularly the generation of other crystalline forms without fusion 

 and the passage of these forms into the granitic, tend to show that 

 it may be produced without fusion. The truth probably is that 

 powerful electro-chemical action can produce all the effects of crystal- 



* In the course of the crystallization of the rock they were probably converted 

 into ores of iron where the iron was in excess. In some walls and parts of walls, 

 and in the lateral radiating zones, where the iron was too thinly diffused to gene- 

 rate proper ores, it would produce a predominance of hornblende, black mica, and 

 other ferruginous minerals. 



