316 FKAGMENTS OF SCIENCE. 



the conclusion that they are the product of a common 



cause. 



But you will ask me how, according to my view, 

 does pressure produce this remarkable result? This 

 may be stated in a very few words. 



There is no such thing in nature as a body of per- 

 fectly homogeneous structure. I break this clay which 

 seems so uniform, and find that the fracture presents 

 to my eyes innumerable surfaces along which it has 

 given way, and it has yielded along those surfaces 

 because in them the cohesion of the mass is less than 

 elsewhere. I break this marble, and even this wax, 

 and observe the same result; look at the mud at the 

 bottom of a dried pond; look at some of the ungrav- 

 elled walks in Kensington Gardens on drying after 

 rain, they are cracked and split, and other circum- 

 stances being equal, they crack and split where the 

 cohesion is a minimum. Take then a mass of partially 

 consolidated mud. Such a mass is divided and subdi- 

 vided by interior surfaces along which the cohesion is 

 comparatively small. Penetrate the mass in idea, and 

 you will see it composed of numberless irregular poly- 

 hedra bounded by surfaces of weak cohesion. Imagine 

 such a mass subjected to pressure, it yields and 

 spreads out in the direction of least resistance; f the 



* I have usually softened the wax by warming it, kneaded it 

 with the fingers, and pressed it between thick plates of glass pre- 

 viously wetted. At the ordinary summer temperature the pressed 

 wax is soft, and tears rather than cleaves ; on this account I cool 

 my compressed specimens in a mixture of pounded ice and salt, 

 and when thus cooled they split cleanly. 



f It is scarcely necessary to say that if the mass were squeezed 

 equally in all directions no laminated structure could be produced ; 

 it must have room to yield in a lateral direction. Mr. Warren de 

 la Rue informs me that he once wished to obtain white-lead in a 

 fine granular state, and to accomplish this he first compressed it. 



