122 MTDBLEMISS : THE GE0L0G3Z OF EDAB STATE. 



Considering this analysis alono. it is evident, from the percentage 

 of silica being jnst over 0(5. that this particular sample belongs to 

 the granite family. On the other hand, considering the percent- 

 ages of K 2 () (5-25). Na 2 (2'66) and CaO (2D5). one would be 

 inclined to place it with the adamellite sub-family but coming 

 near to banatite (the most acid of the mon/.onite sub-family) on 

 one side and potash granite or potash syenite on the other : — 



Potash Granite 



+ 



Adamellite. 



Potash Syenite 



Mon/.onite. 



Inasmuch, however, as this specimen is more basic than the 

 typical Idar rock, containing, as it obviously does, less quart/, more 

 soda-lime felspar and more of the ferro-niagnesian minerals than the 

 latter, it becomes obvious that the more normal Idar rock will be 

 correctly placed among the potash granites. 



One may divide the Tdar granites into the following varieties : — 

 (1) with biotite predominant, (2) miorogra- 



Vaneties of the Q ^ an( j granophyric variety. (3) with horn- 

 granite. , ° l J . „ \ / 



blende in abundance. (4) with topaz (?) and pinite. 



As examples of the first, which is the most common type, I 



may instance the isolated tor, 3 miles S.S.E. 



Biotite granite. , T . _ „ r . n . . . 



of Idar town (sj)ecimen No. 43S . 12114, 

 PI. 14, fig. 1). also the temple hill immediately south of Idar town 

 (No.'^g, 12445) and the rock from 2 miles E.S.E. of Idar. *fo. All 

 these are rather coarse-grained, porphyritic varieties, of pale salmon- 

 pink or creamy pink colour. The commonest form of the potash 

 felspar is microcline, which is in overwhelming proportion to any other 

 felspar. It builds all the porphyritic phenocrysts, which by their 

 colour give the predominant tint to the rock as a whole. These 

 felspar phenocrysts are not very sharply marked off from the ground- 

 mass. Their generally idiomorphic shape is well enough seen, 

 showing that most of these porphyritic elements belong to an 

 earlier generation, but the actual borders and outline are vague 

 and mixed with, or interrupted by, the finer-grained ground-mass, 

 and each phenocryst seems to have included within itself hypidio- 

 morphic quartz granules. Microcline in granites is commonly 



