194 SCIENCE PROGRESS. 



sediments, the author named has mapped three zones of 

 metamorphism, characterised respectively by staurolite, 

 cyanite, and sillimanite. Followed along its strike, a bed 

 which in the outer zone is a staurolite-schist, becomes in the 

 intermediate zone a cyanite-gneiss, and in the inner one a 

 coarse sillimanite-gneiss. These differences seem then to 

 be due to differences of temperature ; and, if it is permissible 

 to press the result to its logical conclusion, the line dividing 

 the cyanite-zone from the sillimanite-zone represents what 

 was an isothermal line during the metamorphism. It is 

 even possible to form some idea of the temperature to 

 which this line corresponds. Vernadsky's experiments have 

 shown that cyanite can be converted into sillimanite by 

 raising it to 1320° to i38o°C. : allowing for the great pres- 

 sure under which the metamorphism was doubtless effected, 

 we may conclude that the temperature reached at the outer 

 edge of the sillimanite-bearing zone was considerably higher 

 than this figure. 



In numerous districts geologists, in studying the 

 "aureole" of metamorphism around a large plutonic mass, 

 have divided it into three or four successive zones or rings, 

 the stages of advancing metamorphism being marked some- 

 times by the incoming of some special mineral, but also by 

 the appearance and disappearance of spotted, foliated, or 

 other structures in the rocks. The results in different dis- 

 tricts do not always bear very close comparison. Such ob- 

 servations as Barrow's, however, would seem to import an 

 element of precision into the subdivision of a metamorphic 

 aureole, and it is therefore very desirable that the interpreta- 

 tion of them should be confirmed by similar study in other 

 regions. Other test-minerals might also be selected : the 

 trimorphous minerals consisting of titanic acid at once 

 suggest themselves. Minute needles of rutile seem to be 

 almost universally present in ordinary clay-slates. In the 

 metamorphism of such rocks the titanic acid is often taken 

 up by new-formed brown mica, or sometimes by micaceous 

 ilmenite or granular sphene ; but in some circumstances the 

 little needles are simply recrystallised as stouter prisms of 

 rutile. Again the rutile-needles may be transformed into 



