Lt.-Gen. McMahon — Conversion of Chlorite. 273 



Whilst this granite was being slowly brought to the surface, the 

 rocks that accompanied it must have been exposed to those potent 

 aqueous agencies which effected so many alterations in igneous 

 rocks, and the traces of which petrologists see in almost every slice 

 of basic rock examined under the microscope. 



The Malvern basic rocks cannot have escaped the ravages of those 

 aqueous agencies. Is it not far more probable then that the chlorite 

 found in those metamorphosed rocks was produced by aqueous 

 agencies after the formation of the biotite by contact action, than 

 that the chlorite was formed before the eruption of the granite ; 

 and, like Shadrach, Meshach, and Abed-nego, escaped absolutely 

 unsinged from the burning fiery furnace of contact metamorphism 

 that converted their fellows from hydrous into anhydrous minerals ? 



I do not think it is necessary to suppose that the chlorite in the 

 Malvern crystalline rocks was derived from biotite ; it may have 

 been derived from some of the other minerals in the rocks. But, 

 on the other hand, the conversion of dark mica into chlorite would 

 not, I think, present much difficulty to a petrologist who knows 

 the story of the conversion of olivine and augite into serpentine. 

 The passage of micas, such as biotite and phlogopite, into members 

 of the vermiculite group (" closely related to the chlorites ") is 

 recognised by Dana (System, 6th Edn. p. 664). If phlogopite, a 

 magnesian mica " near biotite," can be altered into serpentine 

 (Dana, ih. p. 634) there is no difficulty in believing that biotite, 

 or a dark mica allied to that mineral, can be converted, in the wet 

 way, into a mineral of the chlorite, or vermiculite, groups. 



In the case of basic rocks containing magnesian minerals, there 

 would be no difficulty in accounting for the accession of additional 

 magnesia — the magnesian minerals undergoing decomposition would 

 supply that. 



Dr. Callaway gives 34 per cent, as the average proportion of 

 magnesia in chlorite ; and the actual analyses of biotite {vide Dana) 

 give, in some cases, proportions as high as 28 per cent., and in the 

 case of phlogopite as high as 29 per cent. Very little, if any, 

 magnesia would therefore be required for the conversion of a mica 

 rich in magnesia into chlorite. I say "if any," because Dr. 

 Callaway's figures confessedly represent the average proportion. 

 In actual analyses it varies greatly and sometimes falls below the 

 average ; and further the removal of portions of the other bases 

 (a necessary part of the process) would alter the proportion of 

 magnesia to the bases that remained. 



In conclusion I think it desirable, with reference to Dr. Callaway's 

 remarks, to reinsist on the distinction to be drawn between an 

 observed fact and a hypothesis advanced to explain a fact. The 

 fact actually observed is the existence of chlorite and biotite in the 

 same rock. Dr. Callaway explains this by one hypothesis ; I 

 explain it by another. I submit that he is not entitled to allege 

 that the conversion of chlorite into biotite is an observed fact — 

 any kaleidoscopic brandishing of authorities before our eyes not- 

 withstanding — until he has shown that the hypothesis he advocates 



DECADE IV. — VOL. I. — NO. VI. 18 



