446 GEOLOGY, 



phism about to be described, is a foliated or schistose structure (Figs. 365 

 and 366). Even the most massive rocks may be reduced to the foHated 

 form by this process; thus, a granite may be mashed into a gneiss — 

 which is a granite in composition, but has a fohated structure — or a 

 basalt may be converted into a schist, a common term for fohated crystal- 

 hne rocks. Porphyritic rock rendered schistose by pressure is shown 

 in Fig. 366. When massive rocks hke granite or basalt are thus crushed 

 down into the foliated form, the process is in a sense degradational. It 

 is a kind of katamor phism or downward change. It is often difficult 

 to differentiate the schists thus derived by degrading massive rocks, 

 I from those developed by ascensional processes from clastic formations 

 {anamorphism) . The action of heat is important in the evolution of 

 schists of both classes, but the effects of heat may best be taken up 

 where it acts measurably alone. 



Metamorphism by heat. — When a mass of lava is poured out upon 

 the surface, it bakes the mantle-rock which it overruns, in greater or 

 less degree, depending on the mass and temperature. The nature of the 

 effect is much the same as in the process of brick-making, a dehydration 

 of the material, a hardening of the loose matter by the partial wielding 

 of the particles, and sometimes the partial fusion of the surface and the 

 development of new compounds, usually glassy, but sometimes par- 

 tially crystalline. In both the natural and the artificial process, the 

 time element is short, the pressure trivial, and the water action limited. 

 If the heat were to become sufficiently intense, the result would be 

 fusion, i.e., a lava which would solidify into a glass. In such a case, the 

 rock cycle would be carried back to the initial molten state and a new 

 cycle instituted, but this does not usually take place when lava merely 

 overflows the surface. 



If lavas, instead of rising to the surface, wedge in between layers of 

 rock and form sills, or interstratified sheets, the surface above as well 

 ^as that below is baked, and as the excess of heat of the lava can only 

 escape through the neighboring rock, the effects for a given mass of lava 

 are more considerable, and as the time element and the water action 

 (and sometimes the pressure) are usually greater than in the case of 

 extruded lavas, the effects tend rather toward chemical and crystal- 

 line change than to simple baking. This tendency increases with 

 increase in the mass of the lava and in its temperature. Sometimes 

 enormous masses of very hot lava are thrust in between or among the 



