168 ON THE CONCRETIONARY AND 



The foliated structure is distinguished from that 

 properly called laminar, by an undefined, or else a 

 comparatively unlimited divisibility ; and the examples 

 of it are found in the argillaceous schists, in the 

 micaceous schists, in gneiss, and in others of the 

 analogous primary rocks. It is conveniently divided 

 into the foliated strictly speaking, and into the 

 schistose. 



In the former, which occurs in the primary rocks 

 that contain mica, the divisibility is the result of the 

 position of the mineral ; and that position, it is else- 

 where shown, may be the consequence, either of depo- 

 sition or of crystalline polarity. It is unnecessary to 

 dwell on the varieties of aspect which this structure 

 presents ; but these will be found to consist, as in 

 gneiss, in its irregularity and imperfection ; or, as in 

 the finest and flattest chlorite schists, in its extreme 

 tenuity and flatness. The analogous structures which 

 occur in the secondary calcareous or arenaceous strata 

 have evidently resulted from the mode of mechanical 

 deposition by which these have been produced ; and ? 

 very generally, from the conspicuous interposition of 

 very slender portions of clay or of mica. These belong 

 properly to stratification. 



The schistose structure is one of those which may 

 truly be called concretionary ; as it occurs in a homo- 

 geneous rock, and is independent of stratification. It 

 is almost limited to the argillaceous schists ; yet not 

 necessarily to those which are homogeneous, as the 

 mixture of sand, gravel, or fragments, does not prevent 

 its existing in the simpler base by which these are 

 united. A similar structure occurs in the sandstone 

 of Sky, and it will probably hereafter be found in other 

 instances where it has been little expected ; in which 

 case even the secondary strata may often possess a truly 



