":20 HOLLAND: CHARNOCKITE SERIES. 



ordinary eruptives, the consolidation of the more siliceous residual 

 portions of the magma. The old Cornish geologists described these 

 as " contemporaneous veins n to distinguish them from " true veins " 

 which were either distinct subsequent intrusions, dyke-fashion, or 

 valuable mineral lodes filling fissures formed in the previously con- 

 solidated rock. These " contemporaneous " veins are the hystero- 

 genetic Schlieren of Zirkel, 1 and the special form of Schlierengange 

 which Reyer distinguishes as Secret-Gange or Secret-Blatter. 2 The 

 essential point expressed by these terms is the genetic relationship 

 between the acid vein and the rock it cuts, and it was this idea 

 which led to the subsequent replacement of the expression "con- 

 temporaneous veins " by i( segregation veins/' a term which Boase 

 says was introduced by Professor Sedgwick at the suggestion of 

 Whewell. 3 



In the charnockite series these contemporaneous, or segre- 

 gation veins are, in common with those well-known in plutonic 

 masses, usually coarser in grain as well as more acid than the main 

 rock mass which they traverse. Very clear instances are seen in the 

 small hill on the west side of St. Thomas' Mount, Madras, where 

 coarse-grained veins are seen cutting through the type-mass of 

 charnockite (p. 145). The contemporaneous veins only differ from 

 the charnockite in which they occur by this coarseness of grain and 

 the almost complete absence of the ferro-magnesian silicate, hyper- 

 sthene, which characterises the charnockite. The quartz is the same 

 peculiar blue-grey quartz, and the felspar is the same kind of blue- 

 grey microcline, whilst granules of iron-ore occur in both the char- 

 nockite and the slightly more acid contemporaneous veins 



* Lehrbuch der Petrographie, 2nd Ed., 1893, p. 791. 



s Theoretische Geologie, 1888, p. ioi. The term Blatter so often used by Reyer instead 

 of Gange expresses more precisely the form of these bodies which we habitually call " veins " 

 because of the vein-like aspect of their outcrops on any surface. In the same way we speak 

 of lenses because of the lenticular shape of the sections of bodies which are often limited by 

 cylindrical, not spheroidal, surfaces. This additional precision is, however, of little value as 

 long as no confusion is caused by the use of the term " vein ". 



» H. S. Boase. — "A treatise on Primary Geology," London, 1834, p. 355; Sedgwick, 

 Phil. Mag., Vol. IX, p. 284. 



Professor H. Louis adopts this old view of the term •'segregation" in his classification of 

 ore deposits in the 2nd Edition of Phillips' " Treatise " (1896, p. n, foot-note). 



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