IDAR GRANITE, GRANOPITYRE, ETC. 123 



supposed to have been the last mineral to consolidate, ami this 

 in the case of the Idar rocks seems to be true enough for the non-por- 

 phyritic elements, but not so for the phenocrysts in their entire! v. 

 Probably the facts in our case are that the microcline phenocrvsls 

 began to crystallise first, and after attaining a certain size in the 

 pasty magma, finished last as regards their bordering layers. 

 Carlsbad twins are sometimes seen, more particularly in orthoclase 

 when it is present. 



Plagioclase is very subordinate in the more typical biotite- 

 bearing rock. It occurs in small individuals giving extinctions 

 up to 10° and 14° on each side of the twinning line, and therefore 

 is probably albite or basic oligoclase. It is particularly well seen 

 in / 3 :> 5 (12115). Larger amounts in lath-shaped sections are some- 

 times seen, as in 4 2 3 ^ (12447) from Laloda hill, a reddish granite 

 (adamellite) that does not weather into tors. 



Quartz is generally present in fairly large lumps and grains, 

 slightly amethystine-grey in colour, and in sufficient quantity to be 

 typical of granite. Like most other constituents it has no particular 

 outline, being hypidiomorphic (subhedral) everywhere. In a similar 

 but more finely granular, condition it occurs included in the micro- 

 cline. 



Biotite is almost universally present as large individuals, or 

 as a few aggregated together. It has very great absorption, the 

 pleochroism being X, pale yellowish or brownish green, Y and Z. 

 very dark brown to black (basal sections being sometimes distinctly 

 pleochroic, as in 12454), or quite black and opaque, as in 4 2 3 5 8 (12449) 

 from near Laloda. The biotite generally includes iron ores in con- 

 siderable quantity and is spotted in many places, suggesting pleo- 

 chroic halos, though owing to the strong absorption these are not 

 visibly pleochroic. Apatite is also included and in the Asai hill 

 rock, 5 2 fi 8 (12460), is present in very many small prisms. The 

 same is the case with 4 ^ (1244G), from 2 miles E.S.E. of Idar 

 town. Occasionally, as hi 4 a 2 ^ 8 (12453) from the west end of Berna 

 plateau, the biotite is in groups or nests of small individuals aggre- 

 gated together. In this case it is much more generally green in 

 tint than brown and suggests that these aggregations are really 

 small xenoliths. 



Muscovite, though normally absent in all the Idar granites, 

 appears sparingly along with biotite in one example, 4 2 2 r, 7 (12452), 

 from the north end of the Berna plateau. 



