184 HOLLAND: SIVAMALAISER1ES. 



i (&)< Contemporaneous Veins. 



Elseo lite-syenite-pegmatite. 



The prevalent, gneissose, grey variety, darkened by a com- 

 parative abundance of biotite, graphite and iron-ores, is cut through 

 by lighter-coloured, coarse-grained veins composed principally of 

 large crystals of oily-yellow or pink elaeolite and grey microperthite, 

 with occasional flakes of biotite and graphite or lumps of iron-ore. 

 These vary from merely local modifications of the fine-grained 

 variety to definite veins two or three feet wide ; but in all cases at 

 the edges of the veins the crystals interlock across the border, 

 showing a rapid, though not sudden, passage into the commoner 

 type of the rock. Patches of the fine-grained rock are found as 

 autoliths in the coarse-grained veins, and show too an ill-defined 

 junction with the coarse material in which they lie. 



The relation which these coarse-grained veins bear to the 

 finer-grained elaeolite-syenite which they cut is precisely the same 

 as that existing between some acid pegmatites and the finer-grained 

 granites which they traverse. Such veins, distinguished by the 

 older geologists as "contemporaneous veins," were thought to 

 belong to the same general period of eruption as the fine-grained rock 

 which they cut, though naturally there must have been a succession 

 in time between the formation and actual consolidation of the two. 

 Reyer x would regard these veins as a special kind of Schlierengange 

 distinguished under the name Secret-Gange or Secret*B/atter, on 

 account of the supposition that they are formed by the exudation 

 into local rifts of the mother-liquor from the partially solidified 

 rock-mass. Charpentier, when writing of the Pyrenean granite in 

 1823, suggestively described these veins as the *' after-births" of the 

 granite, whilst Carne referred to the similar phenomena in Cornwall as 

 " contemporaneous veins/' distinguishing them from the " true veins," 

 which were either products of a distinct and subsequent eruption, 



1 Theoretische Geologic, 1888, p. 101. 

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