360 Prof. E. H. L. Schicars— What is a Metamorphic Rock ? 



magma are allowed to satisfy themselves without reference to any 

 disturbing factor, and an igneous rock will result. If on the other 

 hand the pressure is not sufficient to produce a complete fl.uid.al phase, 

 in Riecke's sense, but the whole rock remains solid except at the 

 boundaries of the constituent grains where interchange of substance 

 can take place, then the law of least molecular volume comes into 

 play ; the small amount of dissolved substance is restricted in its 

 molecular activity, and a metamorphic rock is produced. 1 As the 

 pressure approaches the static condition the mineral arrangement will 

 tend to become massive like that of igneous rocks, and as the pressui'e 

 decreases the lateral stress will produce more and more pronounced 

 schistose structures. 



If we accept some such definition of a metamorphic rock in terms 

 of molecular energy we can apply it to separate such a rock at its 

 upper limit, as well as at its lower limit, from other varieties. That 

 is to say, a metamorphic rock must be one in which dynamical 

 pressure has acted, and the term cannot be applied to one in which, 

 owing to peculiar circumstances, a mineral usually associated with 

 metamorphism has crystallized, as in the micaceous sandstones referred 

 to above. 



A metamorphic rock under this conception could not arise between 

 two sedimentary rocks, or in the midst of an igneous mass, because 

 the metamorphism, being due to hydro-chemical reactions under 

 dynamical pressure, will be a measure of the 'head' or pressure on 

 the water which permits the chemical activity to proceed. Water 

 under such a pressure will permeate all the surrounding rocks 

 irrespective of their texture or composition, and therefore there can 

 be no selective action on particular zones of the rock-complex, but 

 there will be a progressive intensity of metamorphism from above 

 downwards. Mere accession of heat will not produce a metamorphic 

 rock ; for example, the anorthosites of Canada are traversed by bands 

 of brecciation in which the rock is reduced to a ' llutschmehl ' ; in 

 Madras, in the charnockite gneisses, similar bands or ' trap-shotten ' 

 are produced in which the temperature has risen sufficiently to 

 partially fuse the rock, 2 but these mylonites are entirely different 

 from metamorphic rocks. 



In the case of contact metamorphism the same explanation will 

 simplify considerably the interpretation of the phenomena. Along 

 the course of the dyke, or in the neighbourhood of the intruding 

 boss or laccolite, the water pressure will be temporarily increased to 

 such a degree that metamorphic action can proceed. If the 'head' of 

 the water is maintained either by fresh supplies from the invading 

 magma or by the closeness of the texture of the rock suddenly subjected 

 to the increased water pressure, then metamorphic action will go on 

 vigorously; if, however, the surrounding rocks are not sufficiently 

 compact, or the invading rock is not pumping up sufficient water to 



1 E. Eiecke, " Uber das Gleichgewicht zwischen einein festen, homogen 

 deformierten Korper undeiner fliissigen Phase, insbesondere iiber die Depression 

 des Schmelzpunktes durch Spannung " : Nachr. Ges. Wiss. Gottingen, iv, 

 p. 278, 1894. 



2 T. H. H. Holland, Mem. Geol. Surv. India, vol. xxviii,pt. ii, p. 198, 1900. 



