356 Prof. E. H. L. Sc/ncarz — What is a Metamorphic Rock ? 



in the inner part of the laterite are present in considerable number, 

 but in these slices of laterite they do not appear as abundantly in the 

 outer crust as they do in the inner one ; this, however, may be due to 

 differences in the distribution of micropegmatite in the original rock. 



The microscopical studies have afforded important corroboratory 

 evidence to that supplied by the chemical examinations. Like the 

 latter they indicate that, at any rate in the case of a diabasic rock, 

 the changes which result in the production of a laterite are exceedingly 

 simple, and are such as would result from the mass-action of percolating 

 rain and soil waters through very long periods of time. In the case 

 of the felspar I cannot trace any production of hydrated micas or of 

 kaolinite as intermediate between the plagioclase and the gibbsite. 

 In that of the augite the change appears to be nearly but not quite as 

 simple ; in its case talc appears to be in part an intermediary product 

 directly from the pyroxene, whilst a ferruginous mica seems to be 

 a temporary product from the possible interaction of some of the 

 constituents of the augite and of the orthoclase fraction of the 

 felspars. The fact of the deposition of part of the silica set free 

 by the weathering of the silicates in the form of more or less irregular 

 granules of quartz is fully corroborated. 



The comparisons of the inner and the outer parts of the lateritic 

 crusts indicate that subsequent to the changes which gave rise to the 

 laterite there were changes in the state of aggregation and of the 

 hydration of the oxide of iron due doubtless to its solution and 

 redeposition, whilst indications are not wanting that the gibbsite is 

 subject, but to a lesser extent, to similar changes. 



The examinations have thrown little light on the production of the 

 masses of bauxitic laterite, in which much of the hydrate of alumina 

 appears to be in a colloidal state. 



Y. — "What is a Metamorphic Rock? 



By Professor E. H. L. Schwarz, A.E.C.S., F.G.S., Kkodes University 

 College, Grahamstown, South Africa. 



rpHE latest book on metamorphism, Dr. V. Grubenmann's Kristallinen 

 _|_ Schiefer, still leaves it an open question what a metamorphic 

 rock is. Generally speaking there is no doubt about the matter ; 

 every geologist has a more or less precise idea of what he means 

 by the term, but no one has yet been able to propound a definition 

 which is perfectly satisfactory, and which will enable one to distinguish 

 a metamorphic rock from all other kinds and at the same time convey 

 an expression of the characteristic peculiarities inherent in such 

 a rock. The need of a definition is very necessary. The want of it 

 has led Dr. Grubenmann to include some rocks among the crystalline 

 schists which one ordinarily would not refer to that class, and on the 

 other hand there are some rocks frequently referred to that class 

 which are not included. In the first case, the masses of emery form 

 the twelfth group of Dr. Grubenmann's classification, yet the analysis 

 of the Naxos emery, which reveals traces of boric oxide (1 "15 per cent, 

 in one case) would seem to place these lenses among the ore-bodies 

 deposited by pneumatolitic action. In the second case, the granulites 



