EFFECTS OF METAMORPHISM. 45 



go into details on such a point. We hope to find time to recur to it again after we have described the 

 unstratined rocks. 



7. The insensible passage of schistose into unstratined rocks (gneiss for instance into granite) affords 

 a presumption that the former have been in a semi-plastic state. For all admit the fluidity of granite, 

 either simply igneous or aqueo-igneous. But if we can hardly tell often where the one ends and the other 

 begins, it is fair to conclude that the unstratified have" resulted from the more thorough and complete opera- 

 tion of the same agency that produced the stratified crystalline group. This argument, however, would 

 only show that the schistose rocks have been plastic, but gives us no information as to their previous 

 consolidation. 



We should not have spent so much time on this subject, did it not seem to have a most important bearing 

 on the whole subject of metamorphism. Admit the schists subsequent to their consolidation to have been 

 in a plastic state, by the agency of hot water, steam, and other re-agents, and the whole subject of meta- 

 morphism is easily explained. But deny this, and the phenomena seem inexplicable. 



We resume now a detail of the effects of metamorphism. Several of these, however, have been touched 

 upon in the preceding argument, and will need but little farther notice. The next one to be noticed in 

 particular, has been brought out somewhat fully in the preceding argument, but it needs to be applied in 

 other cases. 



2. A second effect of metamorphism is the abstraction of one or more of the ingredients of 

 rocks and simple minerals by chemical agents. 



If silicious limestone, for instance, should be permeated by water containing some 

 ingredients that would abstract the lime, a porous quartz rock would remain ; or if in 

 a plastic state and under pressure, it would become compact quartz, henceforth the most 

 unchangeable of rocks. We incline to the opinion that this was in fact the mode in which 

 very much of the larger deposits of quartz rock were produced. 



If one or more proportions of oxygen were abstracted from peroxide of iron, or manga- 

 nese, quite different ores would result. In this way have many of the simple minerals 

 and many of the rocks been essentially changed. ' 



3. Similar results, only more complicated, would result from the introduction into the rocks 

 of new ingredients, held in solution by the water diffused through the plastic materials. Hence 

 mineralogists reckon a large number of what they call pseudomorphs ; that is, minerals 

 which have the crystalline form of other minerals, whose cavities they occupy. In this 

 way, too, the character of rocks may be essentially changed. 



4. Though the problem be often quite difficult, yet chemical geologists have been able to point 

 out a great number of these metamorphoses in the rocks with much probability, by comparing the 

 composition of the unchanged with the changed. We give some examples: 



Clay slate has been converted into mica schist, talcose schist, gneiss, and granite. 



The origin of clay slate from clay is obvious to the most common inspection. 



Almost any of the silicious sedimentary rocks can be converted into mica schist. Indeed, hand specimens 

 of micaceous sandstones can hardly be distinguished from mica schist. This rock has also been derived 

 from chlorite schist and from greenstone. 



Mica may be produced from feldspar. That in sandstone was not improbably formed by the agency of 

 meteoric water, subsequent to the deposition of the sandstone. 



Talc, steatite, and chlorite, have been found to result from the decomposition of feldspar, hornblende, 

 augite, garnet, mica, &c. The excess of silica in these minerals may have produced the quartz in talcose 

 and chloritic schists. 



