GEOLOGICAL OCCURRENCE. ' 33 



Himalayan chain — a firm, solid block of land against which the weaker 

 part of the crust has been rolled up by tangential pressures. 



Origin of pegmatites. 



There is probably no other group of rocks whose origin has been 

 the subject of more varied discussion than the pegmatites. De Saus- 

 sure received the support of Credner, Klockmann, Dana, Huntington, 

 Kerr and Sterry Hunt in likening them to metalliferous veins as the 

 result of the successive deposition of mineral matter from solution in 

 fissures, but recent researches support the earlier view of Charpentier 

 (1823) who regarded the pegmatites as injections of granitic material 

 which, originating in the still fluid granite, deep down, was pressed 

 into the cracks of the already solidified granite and rocks above — 

 " after-births," as it were, of the same granitic formation in the dis- 

 trict in which they occur. 1 



Even before Charpentier's time, however, similar views were pub- 

 lished by the old Cornish geologists, Carne, Davy and others, who dis- 

 tinguished between what they called " contemporaneous veins ", which 

 are related genetically to the granite which they accompany and often 

 traverse, and the " true veins ", filled with valuable ores and formed at 

 a distinctly subsequent period by the chemical infilling of fissures. 2 



It is now generally conceded that pegmatites have resulted from 

 the consolidation of injected fluid magmas, often directly traceable to 

 some large granitic mass. This view, that they are merely contempora- 

 neous injections of the residual granite magma, has been advocated by 



1 Charpentier, " Essai sur la const, geog. des Pyrenees", 1823, p. 158. 



2 Carne, " On the relative ages of the veins of Cornwall." Trans. Roy. Geol- 

 Soc. of Corn-wall, II (1822), 49. It is difficult to say who first used the term " con- 

 temporaneous veins ". Dr. John Davy in 1818 (Ibid., I, 20-26) referred to quartz 

 veins traversing the granite of Porth Just as belonging "to that class of veins 

 commonly considered contemporaneous". Those which were formerly called 

 " contemporaneous veins " were in 1834 (Boase, " Primary Geology", p. 355) 

 known as veins of segregation, a term introduced by Professor Sedgwick at the 

 suggestion of Whewell " to express that they have been formed by a separation 

 of parts during the gradual passage of the mineral masses into a solid state". 

 This is the sense also in which the term "segregation" is used by Prof. H. Louis 

 in the second edition of Phillips' " Ore deposits" (1896), p. 11, foot-note. 



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