Alfred Brammall — The Genesis of Chiastolite. 227 



By -virtue of (1) above, whatever be the chemical composition (acid 

 or basic) of the igneous intrusion effecting metamorphosis, the 

 temperature gradient, from the focus to the periphery of the aureole, 

 should traverse a zone within which the temperature appropriate to 

 the production of chiastolite is maintained for a shorter or longer 

 time ; hence, if heat alone (dry or moist) conditions the production 

 of chiastolite in argillaceous rocks, the assertion (met with in most 

 text books) that the occurrence of chiastolite is restricted to such rocks 

 around acid intrusives is without obvious explanation. 



If, however, the production of chiastolite is conditioned by heat 

 and mineralizers carried by water, this restriction receives explanation 

 at once, since the volatile content of acid magmas is notably higher 

 than for basic magmas. The latter (a pneumatolytic) hypothesis, 

 however probable, yet lacks definite experimental support, while on 

 the other hand there is little to urge against the reasonableness of 

 seeking chiastolite where its presence has not hitherto been recorded, 

 that is, in argillaceous rocks altered by contact with basic intrusives, 

 and if a search should prove occasionally fruitful such inconspicuous 

 occurrences of chiastolite might even be regarded as special cases 

 supporting the pneumatolytic hypothesis, in this sense : that as the 

 volatile content of basic magmas is lower than that for acid magmas 

 the purely pneumatolytic effect of the former will be less than for the 

 latter, the ' chiastolite zone ' will be narrower and less distinctive, and 

 the mineral itself less perfectly developed — even only embryonic ; 

 and the occurrence described in this paper is urged as a special 

 instance of the latter character. 



Conclusion. — Similar occurrences elsewhere may have escaped 

 detection for the following reasons : — 



(1) The chiastolite is embryonic — too imperfectly developed to be 



conspicuous in hand-specimens of the parent shale ; and 

 may not have been sought for because its presence was not 

 suspected. 



(2) The zone of production either does not extend to the surface 



of the ground or is limited in width to a few inches ; and 

 even at greater depths the zone may still be very narrow 

 and the mineral embryonic. 



Appendix : Microscopic Features op Shales in Thin Section. 



Shale No. 1. — An umber-coloured almost opaque base of finely divided 

 matter, crystalline in part (since there is polarization, in low order greys and 

 yellows) but of indeterminate nature ; carbonaceous matter abundant ; specks . 

 of sulphidic ore occasional. 



Shale No. 4. — A light almost wholly transparent base, containing but little 

 opaque (? carbonaceous) matter ; pyrites conspicuous, as aggregates^ with 

 radiate structure. Mainly composed of minute irregularly bounded grains of 

 a crystalline substance showing fairly good relief and polarizing in 1st order 

 greys and yellows. Irregularly distributed in the base are patches and strings 

 of minute raggy flakes closely resembling sericite or paragonite. 



EXPLANATION OF PLATE VIII. 

 Figs. 1, 2. — Photomicrographs of thin sections of altered shale parallel to 

 bedding planes taken in ordinary transmitted light. Rhombic, square, and 

 polygonal crystal sections can be made out in each of these figures, and the 

 specific features of chiastolite are also distinctly discernible. 



