120 AUSTRALASIAN ANTARCTIC EXPEDITION. 



this assumption seems to be scarcely warranted from Bastin's paper. Leith and Mead 

 then attempt to subject the criteria to rigorous proof by application to a number of 

 analyses of sericite schists, weathered and hydrothermally altered acid and basic igneous 

 rocks, chlorite schists and hornblende schists. In the tests they fix the critical value 

 of the A1 2 3 content at 5 per cent, excess over the 1 to 1 ratio with lime and alkalies, 

 i.e., with more than 5 per cent, excess Leith and Mead consider that the criterion for 

 sedimentary origin is satisfied, and less than 5 per cent, excess indicates igneous origin. 

 Yet Bastin has stated that a 5 per cent, excess is only sufficient to cause a suspicion 

 of sedimentary origin and that a 10 per cent, excess is necessary to make the sedimentary 

 origin extremely probable. With these interpretations of the meaning of the criteria, 

 Leith and Mead find, as a result of the indiscriminate application to rock analyses, 

 without any consideration of other metamorphic characters, that the chemical criteria 

 have value only when carefully qualified and limited, and that they always fail when 

 other criteria fail. It needs to be added that the applicability of the criteria to 

 weathered and hydrothermally altered igneous rocks is not important, because it has 

 yet to be shown that weathering before the development of foliated structure is not 

 negligible, as assumed by Bastin. 



The problem can be simply stated in terms of metamorphic diffusion and 

 metamorphic differentiation. Chemical composition may be a very important and 

 helpful factor in tracing the history of a schist in those cases in which neither 

 metamorphic diffusion nor metamorphic differentiation has occurred. If Bastin's 

 criteria are satisfied in these cases, the rock is conclusively sedimentary in origin. If 

 the analysis is identical with common and definite igneous rock types there will be strong 

 probability of igneous origin. There will only be doubt where detrital rocks such as 

 tuffs and arkoses and sediments with approximate igneous composition are possible. 



If either metamorphic process is suspected then great caution is necessary. Should 

 there be the metamorphic differentiation of a single oxide like quartz, the remaining 

 oxides in the analysis will still bear the same ratios, and the criteria will avail as in the 

 case of the sericite schist. In general, however, we will be unable to place reliance on 

 these chemical criteria wherever metamorphic diffusion or differentiation has prevailed. 

 The composition of the chlorite rock at Cape Denison satisfies the three sedimentary 

 criteria, the biotite hornblende schist satisfies two of them, while the epidosite does 

 not satisfy any. The criteria are valueless in the case of metamorphic diffusion and 

 differentiation types, because such types possess metamorphic individuality alone. 

 They possess neither the individuality of an igneous rock nor the individuality of a 

 sedimentary rock. They possess a complex history, and have arisen during the 

 metamorphism prior to which they did not exist as individuals. 



The actual extent of these processes, and with it the extent of the limitation of 

 chemical criteria, awaits further research. They do seem to be of more frequent 

 occurrence than hitherto suspected. Yet these processes only have limited range, 

 and therefore in the complete description of any one area we would normally expect 



