Lt.-Gen. McMahon — The Rape of the Chlorites. Ill 



Fig. 4. — Stmirocephalus, cf. Murchisoni, Barr. ; pygidium. x 6. 



5. — Bindijmene Hughesm, Roberts, MS. ; restored figure, x S-j-. 

 6. — Cyphaspis, cf. Raslritum, Tornq.; fragment of head. x 6. 

 7. — Cyphaspis, sp. , cf. Jiimneisteri, Barr.; fragment of head. x 5. 

 8. — Stanrocephaliis (Sphmrocorijphe) iiuicus, Thom. ; fragment of head, x 5. 

 9. — ? Ateleoci/stites ; impression of uppermost median plate, x 1. 

 10. — ? Ateleoci/sfites. x 1. 



Figures 2, 3, 4, 6, and 7, are from specimens found in the Phacops elegans zone=: 

 Stockdale shale series (Valentian) of Wharf e, Yorkshire. 



Figures 1, 5, and 8 are from specimens found in the Bala beds = Coniston lime- 

 stone series of Norber brow. 



Figures 9 and 10 are from specimens found in the Bala beds = Coniston limestone 

 series of Wharf e. 



[The publication of this article has been unavoidably delayed. — Edit. Geol. Mag.] 



III. — The Eape of the Chlorites, 

 By Lieutenant- General C. A. McMahon, F.G.S. 



TWAS not present when Dr. Callaway's last paper on the Malvern 

 Rocks was read before the Geological Society, and had not the 

 opportunity of joining in the discussion that followed the reading 

 of the paper. The author has now given his views regarding the 

 " Conversion of Chlorite into Biotite " in a condensed form in the 

 Geological Magazine (Dec. 1893, p. 535), and has expressly invited 

 criticism on them ; I venture, therefore, in response to this invitation, 

 to give expression to a difficulty that presents itself to my mind. 



It is not necessary to enter into a discussion of the authorities 

 quoted by Dr. Callaway ; but I remark in passing that two of the 

 papers referred to profess to deal with cases of contact metamorphism, 

 and consequently the conditions that must have prevailed in those 

 cases are altogether different from those which govern Dr. Callaway's 

 supposed case of dynamo-metamorphism. In cases of contact actiou 

 one can readily understand how aqueous acid vapours, or liquids, 

 emanating from the molten igneous rock under high pressure 

 penetrated the adjoining rocks, and carried with them in solution 

 some of the constituents of the igneous magma. 



In the case supposed by Dr. Callaway the conditions are altogether 

 different. We have to deal with a consolidated rock which contains 

 the hydrous mineral chlorite, the anhydrous mineral biotite, and 

 others that show a passage between the two. Dr. Callaway contends 

 that the biotite has not been converted into chlorite by aqueous 

 agencies, but that the hydrous chlorite has been transformed into 

 anhydrous biotite through the action of heat. The fact that the 

 rock contains chlorite as well as biotite, and minerals that show a 

 passage between the two, would seem opposed to the acceptance of 

 the author's view; but he meets this difficulty in the following 

 way : — " The reason why in the Malvern rocks we have sometimes 

 a decomposition of hornblende into chlorite, and sometimes a change 

 of chlorite into biotite, is found in the difference of environment. 

 Where the rock is slightly crushed, and there are no signs of rock- 

 fusion, there is decomposition of hornblende ; but where there is 

 intense crushing and shearing, accompanied by a high temperature, 

 reconstruction sets in and biotite is generated." 



