Prof. K H. L. Schivarz—What is a Metamorphic Rock ? 357 



appear to be genetically related to the eclogites and should find a place 

 in a systematic classification of crystalline schists if the eclogites are 

 included, whereas Dr. Grubenmann leaves them out though the 

 eclogites are one of the rock-types included in his fourth group. 



Van Hise logically defines a metamorphic rock as one that has been 

 altered, and hence, as agents of metamorphism, we find listed plants, 

 worms, burrowing animals, and man. One's dinner plate thus could 

 be classed as a metamorphic rock, but it would be an unusual use 

 of the term. There are conventional restrictions to such general 

 terms, just as in the use of the word f geology', which means the science 

 of the world. If we were to apply Van Hise's logical use of terms, 

 then we should mean by geology the science of everything on the 

 earth and the Micyclopcedia Britannica would be all too meagre 

 a textbook. But while the old - fashioned expression ' mineral 

 kingdom' usefully defines the subject-matter of geology, what 

 are the restrictions that are to be observed in the use of the term 

 'metamorphic rock' ? what is the subject-matter to be treated under 

 metamorphism ? 



There is no doubt that the general usage is to exclude from 

 metamorphism the change in rocks produced by weathering and also 

 the change brought about by igneous fusion, whatever that expression 

 means, so that we obtain an upper and a lower limit. 



In the case of the upper limit, what amount of change is necessary 

 in a rock for it to be taken from one of the other classes and placed in 

 the metamorphic class ? Certainly not the ordinary compacting of 

 sediments, nor even the simple cementing of the grains by deposit 

 from solution of silica, iron oxide or sulphide, calcite, etc., will make 

 a sedimentary rock a metamorphic one ; nor will the development 

 of pegmatitic structure take an igneous rock out of its own 

 particular class. 



The development of secondary crystals in sedimentary or igneous 

 rocks is again not sufficient to make a rock a metamorphic one. 

 In the Carboniferous sandstones about Grahamstown the shaly laminae 

 are profusely scattered with scales of secondary mica (sericite), and 

 these lie on perfectly unaltered Devonian shales ; the sandstones have 

 undergone a process of change precisely similar to that which produces 

 a mica- schist, yet the micaceous sandstones are not metamorphic 

 rocks. In the case of igneous rocks in the same way, diabases and 

 melaphyres in which the augites, for instance, are entirely altered to 

 uralite, are still igneous rocks, yet the process of alteration is the same 

 as that which produces a hornblende-schist. 



Dr. Grubenmann defines metamorphism as a process of rock-forming 

 which is quite as characteristic as that which produces igneous or 

 sedimentary rocks ; it impresses on the crystalline schists their 

 peculiar character and invests them with an independence as a class in 

 eontrast to other rocks. But what the ' stempel ' or peculiar character 

 is which, impressed on a rock, at once allows one to class it as 

 a crystalline schist, Dr. Grubenmann does not say. 



The crystalline schists, the same author says, are rocks which may 

 have originated from igneous or sedimentary rocks, or mixtures of the 

 two, in which the chemical composition has remained essentially 



