455 



SHALE. 



UNDER this term are included all the argil- 

 laceous schists of the secondary class, sometimes 

 also known by the inconvenient and awkward 

 name of slate clay. 



These have seldom been found occupying 

 extensive tracts in nature, and it is even rare to 

 find them in considerable beds. The Orkney 

 Islands are however an exception to this general 

 rule. They are commonly in the form of thin 

 strata, often of mere laminae, alternating with the 

 other rocks with which they are associated. 

 Hence they are often implied by geologists, where 

 they are not mentioned in any series, or forma- 

 tion ; being considered as subordinate substances. 

 They present, in this case, a striking analogy 

 to the argillaceous schists of the primary class, 

 which often occur in a similar manner among 

 the other rocks of that division. 



