358 Prof. E. H. L. Schwarz—Wliat is a Metamorphic Rock ? 



unaltered. Consequently rocks of the same composition, no matter 

 what their origin, under the same conditions of metamorphism, yield 

 the same variety of crystalline schist. That is to say, a sandy 

 dolomite with a layer of salt on top to render sodium to the mass, 

 will, under the action of metamorphism, produce a diorite gneiss just 

 as readily as an original diorite of approximately the same composition. 

 Still, the amount of alteration necessary to change the sedimentary or 

 igneous rock into a metamorphic one is left unexpressed. 



If we take Dr. Gruhenmann's three types of metamorphic rocks we 

 seem to be near the possibility of a definition ; there are, according to 

 this author, rocks of three types, those belonging to the uppermost, 

 middle, and lowermost zones of metamorphism, the first being 

 represented by the phyllites and chlorite schists, the second by the 

 schists proper (mica-, hornblende-schists), and the third by the gneisses 

 and eclogites. The zones are marked off by the action of pressure. 

 In the uppermost zone the pressure is stress (mass X motion) or, as 

 we might call it, dynamical pressure ; in the lowermost zone the 

 pressure approximates to hydrostatic pressure, or pressure acting 

 equally in all directions, which we can call static pressure. Under the 

 action of these forces hydro-chemical processes have gone on in the 

 sense that in the uppermost zone the compounds formed obey the volume 

 law, that is to say, are those with the smallest molecular volume ; in 

 the lowermost zone the activity of the molecules is such that they are 

 enabled to overcome the pressure from without and form compounds 

 independently of the volume law. Thus, an impure dolomite might 

 yield in the respective zones — 



Lowermost zone. 

 Augite and Molecular 



anorthite. volume. 



CaMgSi a Oo . . . 68-0 

 CaAljSiaO.s . . 101 "1 



145-8 



169-1 



Under extreme pressure, where static pressure is alone active, the 

 crystals will form without any reference to direction, which is produced 

 by dynamical pressure, hence the minerals in the crystalline schist will 

 crystallize as in an igneous rock. Here we come to the difficulty in 

 defining the lower limit of crystalline schists ; for if it be true that 

 an increasing disregard of the volume law is characteristic of the lower 

 zones, then the igneous rocks become simply metamorphic rocks in 

 one stage lower than Dr. Grubenmann's lowest zone, thus : — 



Lowermost metamorphic zone 

 Eclogite. 



Garnet, 3K 3 Al 2 Si 3 0i2 

 Omphacite, Na Al Si 2 Oc 

 Quartz, 2 Si 2 



479-4 



550-4 



