ARGILLACEOUS SCHIST. 189 



posed ill beds of various dimensions; under the usual 

 variations of dip and direction. These arc also sub- 

 ject to flexures, yet seldom displaying such intricate 

 contortions as micaceous schist or gneiss, except when 

 in contact with granite or trap, or traversed by veins 

 of these substances. Moreover, the laminae are some- 

 times bent or contorted, even when the beds are re- 

 gular and their planes parallel. To a cylindrical or 

 conical flexure, occurring in these beds and in the other 

 primary schists, the term mantle-shaped has been ap- 

 plied by those who find strange virtues in words. This, 

 and other such terms, I have designedly avoided ; re- 

 serving also the only remark which they require, to 

 this place. All the primary strata generally preserve 

 an uniformity of direction and inclination, unless gra- 

 nite be present, in which case they put on these forms, 

 and commonly in proportion to their proximity to this 

 rock. The causes must have long since been obvious. 



In alternation with other rocks, it sometimes occu- 

 pies very small spaces ; a single bed occurring among a 

 large mass of strata, not to appear again throughout a 

 large portion of the series, being also variable in breadth 

 and persistence, and sometimes a mere lamina, exte- 

 nuated to nothing after a very short course. Occa- 

 sionally, many are accumulated in depth, without la- 

 teral persistence; thus forming insulated and irregular 

 masses, not unlike the independent deposits among the 

 secondary strata. In other instances, it occurs in con- 

 siderable tracts, containing occasional beds of other 

 substances : or in a long succession of alternations 

 with one or more others; forming, lastly, an occasional 

 member in a very irregular and various series ; as will 

 presently be noticed. 



With respect to the positions of this rock towards 

 the primary, it immediately follows granite in Cornwall, 



