PETROGRAPHY 359 



Ardennes, and of Lapworth on the north-west Highlands, 

 revealed many new and highly interesting subjects of research 

 in connection with dynamo-metamorphism. 



Johann Lehmann, in the work mentioned above (p. 355), 

 accepted the views of Lessen, and demonstrated the effects 

 of " dislocation metamorphism " from a large number of 

 excellent illustrations of microscopic rock-sections. Accord- 

 ing to Lehmann, the metamorphic rocks may be arranged in 

 two groups. One comprises the various modifications of 

 gneiss, granulite, felsite, and hornblendic schist which have 

 originated as rock-material consolidated from molten magma, 

 and have received their characteristic foliate structure from 

 the action of pressures before solidification had been com- 

 pleted. He advocated the plutonic origin of this group upon 

 the assumption that there is in the crust a corresponding 

 rock-magma, the source of the deep-seated eruptive rocks, 

 granite, syenite, diorite, gabbro, and that these rocks are con- 

 nected with the gneiss group by a complete series of transitional 

 modifications. 



The other group comprises the remaining crystalline schists, 

 gneissic schist, micaceous schist, chloritic schist, talcose schist, 

 phyllites, etc. These have been produced by "dislocation 

 metamorphism " carried out in very high degrees. In the 

 case of gneissic schist the original rock-material, while under- 

 going the processes of metamorphism, has been invaded 

 by, or impregnated with, granitic injections, but the series 

 of typical schists have been metamorphosed without any 

 injection of foreign magma. The original character of the 

 rock-material is, according to Lehmann, not always demon- 

 strable, but he thinks it abundantly evident that the meta- 

 morphic series is intimately associated in the field both with 

 fragmental or clastic deposits and with rocks of igneous 

 origin. Lehmann insisted that it was erroneous to attribute 

 the metamorphic schists to a definite, pre-Cambrian geological 

 epoch ; it was in his opinion far more probable that they 

 belonged to the different epochs during which extensive 

 mountain-movements had been in progress. Professor Barrois 

 in 1884 likewise showed that the schists and gneisses in 

 Brittany, which had been regarded as pre-Cambrian, really 

 represented metamorphosed sedimentary deposits belonging to 

 various Palaeozoic epochs. 



The involved stratigraphical problem presented by the 



