Prof. E. H. L. Schwarz—What is a Metamorphic Rock ? 359 



It is very generally held now that igneous fusion of rocks is 

 accomplished with the help of water ; whatever rock we examine, 

 whether it belongs to the acid series with micas which contain 

 combined water, or to the basic rocks (perhaps with the exception of 

 some of the ultra-basic rocks), there is always evidence that a part 

 of the peculiarities exhibited by it must be explained by aqueous 

 solution. Quartz is readily soluble in water at 200° C, and becomes 

 a powerful acid like nitric or sulphuric acid ; the solution becomes 

 a solvent in which bases can be dissolved, and on cooling or super- 

 saturation these would combine with the silica to form the various 

 minerals of igneous rocks. The pegmatites and quartz veins which 

 come from granite masses can be regarded as the end products 

 of a series of crystallizations which began by the formation of the 

 normal granite and which were accomplished by hydro-chemical 

 processes. There is the old experiment of Pouque and Michel-Levy 

 of fusing microcline and biotite ; the melt on crystallizing gaveleucite, 

 olivine, and magnetite. If it be objected that the pressure on the 

 earth's crust was the only thing needful to keep the chemical 

 combination represented by microcline and biotite, we have Oetling's 

 experiments to contradict such an assumption. Oetling, 1 on fusing the 

 rock-forming minerals under pressure, found that the melt solidified 

 as a glass, whereas under ordinary pressure they crystallized, somethues 

 as the original minerals, sometimes, as in Fouque and Michel-Levy's 

 experiment, in other forms. Whichever way we attack the question 

 we find that solvent water is the agent active in forming the character- 

 istic minerals and structures of igneous rocks: a discussion of the whole 

 question, with references, is given in Natural History of Igneous Rocks, 

 by A.. Harker, pp. 294 et seq. 



If this be so, then where are the dividing peculiarities between, 

 say, a graphic granite and an eclogite ? Both form dyke-like masses 

 which may be in both cases in gneiss ; both are noncrystalline with 

 the crystals formed without reference to any leading direction, and 

 more than one mineral may be common to both (quartz, felspar, 

 hornblende, etc.), and yet the one is unquestionably a rock belonging 

 to the so-called igneous class and the other to the metamorphic class. 

 The answer seems to me to lie in a recognition that in the igneous 

 rock some law of chemical combination has been carried out under 

 circumstances which have allowed complete freedom of molecular 

 action, whereas in the metamorphic rocks there have been restrictions. 

 Using the fact that the sum of the molecular volumes of the minerals 

 in an igneous rock are greater than the sum of the molecular volumes 

 of corresponding minerals in a metamorphic rock of the same chemical 

 composition, it is perhaps possible to define the differences in the two 

 as due, in the one case, to the internal pressure (i.e. molecular 

 activity) having been greater than the external pressure, and in the 

 other that the reverse has obtained. In other words, if a rock magma 

 is under sufficient pressure to allow the solid particles to move freely 

 into the solvent water, the chemical affinities of the substances in the 



1 A. Oetling, " Vergleichende Experimente liber Verfestigung gesehmolzener 

 Gesteinmassen unter erkoktem unci normalem Druck" : Tscher. Mitt., xvii, 1898. 



