62 Dr. Mac Culloch on the Concretionary and 



It is conveniently divided into the foliated strictly speaking, and 

 into the schistose. 



In the former, vyhich occurs in the primary rocks that contain 

 mica, the divisibility is the result of the position of this mineral ; 

 and that position may be the consequence either of deposition 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 geologist ought to be careful not to confound it with 

 those appearances which occur in the secondary calcareous or 

 arinaceous strata; which, although strictly laminar forms, have 

 evidently resulted from mechanical deposition, and often from the 

 conspicuous interposition of very slender portions of clay or of 

 mica. 



The schistose structure is one of those which may truly be called 

 concretionary; as it occurs in a homogeneous rock, and is inde- 

 pendent of stratification. It is limited to the argillaceous schists ; 

 yet not necessarily to those which are homogenous, as the mixture 

 of sand, gravel, or fragments, does not prevent its existing in the 

 simpler base by which these are united. I must here, however, in- 

 terpose an exception, not well knowing where else to place it, re- 

 specting a peculiar structure occurring in some sandstones, which 

 is neither rigidly laminar, nor properly foliated or schistose. It is 

 the complicated case of the sandstone of Sky, described in the 

 account of the Western Islands, and it is probable that it will here- 

 after be found in other instances where it has been little expected ; 

 in which case it may appear that even the secondary strata may 

 often possess a truly schistose or laminar structure, where the ap- 

 pearances have been attributed to stratification. 



The schistose concretionary structure is not necessarily straight, 

 but is sometimes found to be curved, as in clay-slate ; and that 

 the curvature belongs to the structure, and not to the bed, is evinced 

 by the regularity or evenness of the latter. 



It is possible that this circumstance may tend to explain some of 

 the complicated curvatures that occur in beds of micaceous schist 



