207 



ally exfoliating, on the exposed surfaces, in irre- 

 gular laminae. The fracture is even, or imper- 

 fectly conchoidal, and splintery also. When 

 long exposed to the weather, it assumes a 

 granular appearance. It has not the waxy as- 

 pect of compact felspar ; which however it much 

 resembles, and into which it appears to pass by 

 an undefinable transition. On the other hand, 

 it becomes more earthy, and passes into clay- 

 stone. Under these different circumstances, it 

 yields with more or less difficulty to the knife. 

 It is not necessarily or exclusively sonorous, as 

 its name would imply. That property is pos- 

 sessed by many other rocks in a much higher 

 degree, and very often depends more on the 

 form than the material. The colours are very 

 various. 



The indurated clays are of different degrees 

 of hardness, and vary in colour, from nearly 

 white to black. They might, according to the 

 name, be confounded with some of the argil- 

 laceous schists ; but they are almost always dis- 

 tinguishable by breaking with the same facility, 



