RESEARCHES IN THERMAL METAMORPHISM. igi 



minerals. But in the processes by which such a rock furnishes 

 the material for sediments, chemical as well as mechanical 

 degradation plays a part ; and, in particular, the soluble be- 

 come separated from the insoluble products of alteration ; 

 the alumina, in whatever combination, remaining among the 

 latter, and so becoming relatively more abundant. Recrys- 

 tallisation of the derived sediment by thermal metamorphism 

 must therefore give rise to new combinations, including 

 some mineral rich in alumina. 



Andalusite and its variety chiastolite have long been 

 known as characteristic metamorphic products in altered 

 shales and slates ; but it has only recently been recognised 

 that cordierite plays in some districts a similar part. The 

 widespread occurrence of this mineral as a "contact" pro- 

 duct was first made known from the Saxon area described 

 above, but it is found under the same conditions in other 

 parts of the world, and it is possible that further examina- 

 tion may discover it in some districts where it has been 

 overlooked. 



In Japan, cordierite is known in several localities. It 

 was noted by Hussak in 1883 from the Asama-yama dis- 

 trict, but recently a more remarkable occurrence has been 

 described by Kikuchi (7) from the Waterase-gawa. The 

 mineral occurs there as a " contact-mineral " in dark Palaeo- 

 zoic slates, and is remarkable not only for the large size and 

 frequent freshness of its crystals, but also for their peculiar 

 structure. Besides the usual polysynthetic twinning, giving 

 a hexagonal cross-section with division into six sectors, each 

 crystal shows an inner and an outer portion. The division 

 between the two portions shows as a hexagon in transverse 

 sections and as two diagonal lines in lono-itudinal sections. 

 These and the junctions of the several twinned individuals 

 are marked by accumulations of inclusions, mostly of car- 

 bonaceous matter. The mineral thus presents a distinct 

 variety of cordierite corresponding with the chiastolite 

 variety of andalusite. 



Some approach to this structure is seen in cordierite in 

 the metamorphosed Skiddaw Slates of the Caldew Valley in 

 Cumberland (8), but the crystal-grains are there much 



