ON THE FOEMATION OF CRYSTALLINE ROCKS. 297 



means of compression, not onl}' rolling-mills, but also the lever presses 

 moved by steam, which are used to stamp sheet iron into various 

 utensils. All these methods of compression, whether gradual or by 

 shocks, were successively employed. The matter upon which I prin- 

 cipally operated was clay brought to a peculiar state of desiccation.* 

 The clay submitted to these different processes of compression 

 acquires a very marked schistose structure; t but for that, besides 

 pressure, two other conditions are indispensable. 



1st. It is necessary that the substance be capable of sliding and 

 extending itself by an inchoate lamination; then the lamelhe develop 

 themselves parallel to the sides — that is to say, normally as regards 

 the pressure. No result is obtained if the body cannot yield and 

 change its form in the direction perpendicular to the pressure. A 

 piece of clay, cylindrical in form, placed in a cylinder of cast iron of 

 the same form and dimension, was powerfully compressed by a piston 

 of the same calibre. The substance acquired very great consistency, 

 but showed no indication of lamellas, not even of cleavage. This, I 

 repeat, takes place only when the mass of earth fits exactly in its 

 rigid envelope; otherwise deformation takes place, and consequently 

 the formation of lamelltB. 



2d. The substance to be compressed ought to possess a high degree 

 of plasticity. Too dry, it breaks; too soft, it becomes laminated 

 without any lamellar structure being perfected. Specimens of the 

 same clay in different states of dryness, submitted simultaneously to 

 compression, furnish superposed layers, some of a schistose structure, 

 others of an irregular fracture, whose contrast is very significative. 

 I have tried also to produce by the same process the schistose struc- 

 ture of the silicates at the moment when they pass from a state of 

 fusion to a solid state. The slags of iron blast furnaces which I sub- 

 mitted to pressure while they were still in a state of paste did not 

 become schistose. The fracture of the cooled mass only showed dif- 

 ferent colored veins in lines normal to the pressure. What I -have 

 said above, concerning the influence of plasticity in the formation of 

 the lamellge, explains the frequent transitions which we see in the 

 same mass of partially schistose rocks. It is thus that, to cite an ex- 

 ample, the porphyry of Mairus in the Ardennes, becomes gradually 

 schistose. 



It is proper here to call to mind that glass acquires a very remark- 

 able schistose structure from causes entirely different from those 

 which we have just mentioned. At the same time that the glass tube 

 transformed by superheated water swells considerably it takes this 

 schistose structure in a very marked manner. The lamellaj into 

 which it easily cleaves, are so thin that sometimes more than ten can 

 be distinguished in a thickness of the sixteenth of an inch.:|; When 



"These experiments took place at the factory of Karcher and Westermann, at Ars- 

 snr-Moselle. I used especially the refractory clay of Villy-en-lYode, (Aube,) which comes 

 from the neocomian formation and from the grey supraliasic marls. 



•j- Certain tiles obtained by a peculiar process of moulding which is employed at Epinal, 

 also often present a commencement of the foliated structure. 



J Sometimes they can be as neatly detached the one from the other as the leaves of a 

 quire of paper. 



