I06 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [May 1 894, 



such general acceptance. These papers are too exclusively minera- 

 logical to be noticed at any length on the present occasion, relating, 

 as they do, principally to the production of secondary minerals at 

 shear-zones, and discussing difficult problems in connexion with 

 change of substance on a large scale in rock-masses (metasomatosis). 

 The chief chemical and mineral changes, he says, have taken place 

 in bands of rock which have been subjected to a shearing movement, 

 so that the metamorphism may be described as • zonal.' The maxi- 

 mum of alteration has been produced in diorite where it has been 

 sheared in proximity to granite -veins. It is considered that contact 

 effects are here combined with dynamic metamorphism. Amongst 

 the most important chemical changes are the removal of bases and 

 the combination of potash with some of the constituents of diorire. 

 The chief mineral changes are stated to be the reconstruction of 

 felspar, and the production of biotite through chlorite from horn- 

 blende ; of white mica from orthoclase, plagioclase, black mica, and 

 chlorite ; also of granular quartz, sphene, and actinolite. Dr. Cal- 

 laway is likewise prepared to go a long way in the direction of 

 wholesale substitution. The appearance of sedimentary rocks 

 towards the south end of the chain, so often urged by Mr. Eutley, 

 is quite delusive ; all this is the result of more intense shearing. 

 The more highly quartzose the rock, the more intense has been the 

 metamorphism. Even a diorite may have all the bases squeezed or 

 dissolved out of it, and thus figure in the end as an acidic schist. 

 The gradual elimination of the magnesia, a necessary accompani- 

 ment of this operation, is one of the points to which attention is 

 drawn in the last paper. It would have been more to the purpose 

 if the Author could have shown us how to get rid of the alumina. 



Reverting to the main question, I am not aware that there is any 

 good field-evidence as to the existence of shear-zones at all in this 

 region, though mere shear-planes, where the parallelism is more or 

 less accidental, are stated by Dr. Irving to be conspicuous structural 

 features of the rock-masses exposed to view in extensive quarries 

 at North Malvern and elsewhere. 1 It has been hinted that the 

 crushed material in the so-called shear-zone at West Malvern may 

 perhaps be a ' friction breccia,' and the possibility of other supposed 

 zones being mere dislocations has also been indicated. Altogether 

 we must admit that Dr. Callaway, by his three papers in the 

 Quarterly Journal, has made a material contribution to our know- 

 ledge of the origin of the Malvern crystallines, besides having raised 



1 Geol. Mag. 1892, p. 461. 



