ARGILLACEOUS SCHIST. 349 



of the latter, or the gray wacke schists, are suffi- 

 ciently divisible for ceconomical purposes. 



In most cases, these laminae are indefinitely, 

 although imperfectly, divisible ; so that the entire 

 structure of the stone is schistose, or nearly scaly ; 

 but, in others, k appears limited to some defi- 

 nite dimension, so as to afford examples of that 

 which, in the 8th chapter, is distinguished from 

 the schistose structure by the term laminar. 



In general, the schists are flat, being some- 

 times also smooth ; but, in others, they are mi- 

 nutely undulated. In some rare instances, how- 

 ever, an example of which may be seen in the 

 island of Luing, the laminae are bent while the 

 strata themselves are straight. This occurrence 

 indicates that the laminar disposition is not inva- 

 riably the consequence of the mode in which the 

 rock was deposited ; a fact which is further 

 proved by the following circumstances. 



Although the fissile tendency is often parallel 

 to the plane of stratification, it will sometimes be 

 observed to lie in an oblique direction to that ; 

 proving, as in the former case, that it is the re- 

 sult of a concretionary structure. A more cer- 



