part 4] PETROLOGY" OF THE ARKAGE DISTRICT. 463 



containing a considerable percentage of the albite-molecule — a 

 soda-microcline. Orthoclase is not abundant in this type : it 

 occurs as grains of large size in certain specimens. Large grains 

 of micropegmatite are noted in the rocks rich in alkali-felspar; 

 these grains are either enclosed in the felspar, or lie at the borders 

 of two felspar-grains. 



Cordierite forms euhedral prisms, with well-developed trillings 

 and magnificent pleochroic haloes around zircons. It is often 

 enclosed by both felspar and quartz ; but, from its characters, 

 relation, and form, there can be no question that it is a true 

 separation from the magma, and not xenocrystic from some pre- 

 existing cordierite-bearing rock. In size it may reach 5 mm. or 

 more in length ; its haloes are often 025 mm. across. Occasionally 

 it includes small biotite-flakes or green spinel-grains. It alters 

 marginally and along cracks, either into yellowish serpentinous 

 substances, or into micaceous products, which in later stages pass 

 into individualized colourless mica-plates. 



Biotite forms large plates, Avith a pleochroism f rom pale 3'ellow- 

 brown to deep red-black. These plates have often a tendency to 

 finger out into the adjacent minerals, or to show a subskeletal 

 form. They alter to green chlorite, exposing a delicate sagenitic 

 Aveb. 



Garnet is of widespread occurrence, and builds pink subhedra 

 often 1 cm. across ; it encloses quartz-grains and biotite-flakes. 

 Spinel forms clusters of deep-green grains enclosed in cordierite- 

 patch.es. Sillimanite has been noted once, and then builds small 

 prisms. Blue tourmaline is often seen; in one example it 

 builds a micropegmatitic intergroAvth with quartz between felspar- 

 crystals. Iron -ore is not abundant, and forms black rounded 

 grains usually associated Avith biotite. Zircon is of widespread 

 occurrence ; apat ite is less common. 



The relative proportion of the chief constituents is extraordinarily 

 variable. Any of the four main components (quartz, cordierite, 

 felspar, and biotite) may dominate, and single hand-specimens may 

 show bands of very diverse ratios. Perhaps the main type, 

 howeA r er, is one in which there are about equal amounts of quartz, 

 cordierite, oligoclase-andesine, and biotite, Avith a few grains of 

 soda-microcline and a spattering of garnet and spinel (see fig. 4). 

 Two rocks of this type have been analysed, and these are included 

 in Table III, Analyses VI & VII (p. 464). Occasionally, perhaps 

 80 per cent, of the rock may be formed of large crystals of 

 cordierite, between which are grains of quartz, oligoclase-andesine, 

 and a little biotite. Sometimes, on the other hand, cordierite may 

 be absent, and the rock is composed of quartz, oligoclase-andesine, 

 and biotite, of what is usually taken as a purely igneous aspect. 

 Again, the relative amounts of the tAVO main felspars varies ; 

 oligoclase-andesine is always present, while soda-microcline is often 

 absent, but in some examples this latter felspar may be quite as 

 abundant as the former. Quartz-rich varieties are common, and 

 form granitic bands in the other variations. All these types occur 



