THE METAMORPHIC ROCKS OF ADELIE LAND 8TILLWELL. 119 



maintained that precisely similar amphibolite rocks could be derived from both altered 

 limestones and igneous rocks, and that there was no alternative. While we remain 

 unconvinced in this respect this difficulty is not acknowledged. 



Trueman* also discusses chemical criteria as well as the criteria of texture and of 

 zircon grains. He finds that texture is not successful in determining origin, but he 

 demonstrates the use of zircon grains, though this method can have only limited 

 application. Trueman describes the alteration of quartzite into sericite schist 

 at Waterloo, Wisconsin. The application of the zircon criteria confirms the chemical 

 work of J. H. Warner, and shows that all gradations exist between the normal quartzite 

 and the most highly developed sericite schist. The zircon criteria demonstrate that 

 the sericite schist cannot represent a former argillaceous layer in the quartzite, nor has 

 the change of composition been affected by the introduction of igneous material from 

 without. With the development of the sericite schist he finds a simultaneous removal 

 of silica, and he notices the presence of quartz stringers in some of the bands. The 

 observation might, perhaps, be stated that the sericite schist with quartz, as a 

 metamorphic differentiation product, is formed during the metamorphism of a quartzite 

 under special conditions. Dwelling on the composition of the platy minerals that 

 develop under the special conditions, he is inclined to attach small importance to 

 Bastin's criteria, even though they are satisfactory in his particular case. The actual 

 transfer of material is the difficulty to Trueman, whereas Bastin formulated his criteria 

 on the supposition that no essential change occurred in chemical composition during 

 the metamorphism. 



Bastinf replies to Trueman's point, and further demonstrates the value of his 

 criteria in special cases when used as an adjunct to textural and structural evidence. 

 He then turns to the question of transference of material during metamorphism and 

 acknowledges that the value of chemical criteria is part of the broader question of the 

 actual extent to which transfer of material takes place. 



Leith and Mead have devoted considerable space to this question in their recent 

 publication^ titled " Metamorphic Geology." These authors emphasize that the use of a 

 chemical analysis in this respect depends on two fundamental assumptions, viz., (1) that 

 the rocks before rock flowage (metamorphism) had a distinctive composition sufficient 

 to identify them as sediments ; and (2) that there is no essential change in composition 

 during metamorphism. We think it will be generally admitted, with Bastin, that 

 sediments do have, in the great majority of cases, a distinctive chemical composition, 

 and that the crux of the problem lies in the second assumption. 



At the outset of their discussion Leith and Mead assume that the criteria set forth 

 for sedimentary origin by Bastin may be used conversely to prove igneous origin. But 



* " The Value of Certain Criteria for the Determination of the Origin of Foliated Crystalline Rocks," J. D. Trueman. 

 Joura. GeoL, vol. XX., 1912, pp. 229-258, 300-315. 



t " Chemical Composition aa a Criterion in Identifying Metamorphosed Sediment*," E. S. Baitin, Journ. Geol., vol. 

 XI., 1913, p. 193. 



J " Metamorphic Geology," C. K. Leith A W. J. Mead. Henry Holt A Co., New York, 1915, p. 226, el teg. 



