RESEARCHES IN THERMAL METAMORPHISM. 195 



little crystals of anatase, as found by Hutchings in the 

 Coniston Flags near the Shap granite (3), or of brookite, as 

 described by Beck (6), (10) in the slates of the Elbe Valley. 

 These latter, purely paramorphic, changes, however, seem 

 to occur locally and somewhat capriciously, so that it is not 

 yet possible to refer the production of one or other of the 

 three forms of titanic acid solely to the effect of higher or 

 lower temperature of metamorphism. Further study in 

 this line may perhaps lead to some more definite conclusion. 

 An interesting result of such researches as those of Miss 

 Gardiner and of Barrow in Scotland is the clear evidence 

 that thermal metamorphism of sedimentary strata may give 

 rise to thoroughly typical gneisses, some characterised by 

 such minerals as garnet, sillimanite, etc., but also in many 

 examples rich in felspar. The abundant occurrence of 

 felspars of various kinds in thermally metamorphosed sedi- 

 ments is now established by the observations of many 

 petrologists. The apparent reluctance with which some 

 have admitted it is perhaps partly due to the difficulty of 

 identifying the minerals when they occur in a fine-grained 

 mosaic. Such varieties as orthoclase, albite, and oligoclase 

 naturally occur plentifully in the metamorphism of such 

 slates, flags, and greywackes as had not been greatly im- 

 poverished in alkalies in the processes by which they were 

 derived from felspathic crystalline rocks ; and in these the 

 aluminous silicates may be only feebly represented. Facts 

 drawn from the study of thermal metamorphism cannot fail 

 to have an important bearing on the problem of the gneisses 

 and other crystalline rocks which constitute the much- 

 debated " Grundgebirge" of large tracts of country in 

 Europe and elsewhere. It is becoming increasingly mani- 

 fest that under such a name as " gneiss " have been included 

 rocks having quite diverse modes of origin. Some are 

 doubtless igneous rocks, with structures either original or 

 impressed on them by dynamic metamorphism ; but others 

 are to be regarded as metamorphosed stratified rocks of 

 various types. It has already been remarked that the occur- 

 rence of special characteristic minerals may throw light on 

 the origin of such rocks in some cases. Chemical analyses may 



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