part 4] PETROLOGT OF THE ARNAGE DISTRICT. 477 



alkali-felspar in less amount. The cordierite in some specimens 

 forms perhaps 97 per cent, of the rock ; it builds patches or 

 grains, and is repeatedly twinned, often with the production of 

 fantastic chequered patterns; it shows good haloes around zircons; 

 its decomposition-products are pale micas. Spinel occurs as 

 green angular grains in swarms in the cordierite, or associated 

 with sillimanite ; these grains occasionally reach 0*5 mm. in 

 diameter. Sillimanite is common ; it forms long needles, some- 

 times so abundant that the slice shows a dense matted felt in- 

 terrupted by cordierite-grains (see fig. 6, p. 476) ; another mode 

 of occurrence is as large aggregate prisms made up of parallel 

 oriented smaller prisms, the large aggregate often measuring 

 0'5 cm. across. Garnet forms pink spongy aggregates enclosing 

 quartz and biotite. Acid plagioclase, both oligoclase and acid 

 andesine. is not abundant, and occurs as grains often speckled 

 by other components. Some alkali-felspar is noted in certain 

 examples. Quartz is common in some of the banded types. 

 Biotite is never abundant, and is the usual hornfels-biotite, 

 pleochroic from pale yellow to deep rich reel. A noteworthy 

 constituent of some examples of this t} r pe of hornfelses is hyper- 

 sthene, which forms rounded, vividly pleochroic grains of small 

 size, associated with quartz, biotite, and plagioclase. 



The dominant hornfels of this class is one composed mainly of 

 cordierite, with quite subordinate amounts of spinel and sillimanite 

 and a few grains of felspar and quartz. The rock, the analysis 



Table IV. — Analysis of Cofdierite-Hornfels. 



IX. 



SiO., 45-85 



TiO~ 1-15 



A1 2 3 20-50 



Fe,,0 3 11-91 



Cr„0., 



VA 



FeO 11-32 



MnO 0-26 



(CoNi)O nt.fd. 



BaO nt.fd. 



CaO 1-27 



Mg-0 4-18 



K 0-72 



Na.,0 1-27 



Li,,0 nt.fd. 



H.,Oat 105° C 0-07 



H'O above 105° C 1-48 



P.,0 5 0-04 



FeS.', 0-07 



Fe T S, 



CO., nt.fd. 



Total 100-09 



IX. Cordierite-spinel-liornfels, Gallowhill Brae, Arnage. 

 Anal. E. G. Eadley (see p. 478). 



