CRYSTALLINE STRUCTURES OF ROCKS. 169 



schistose or laminar structure, where the appearances 

 have been attributed to stratification. 



The schistose concretionary structure is not neces- 

 sarily 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 regu- 

 larity 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 under similar circumstances ; but the suggestion 

 was reserved for this place, as it seemed that its value 

 would here be better understood. But that all cur- 

 vature is not of a concretionary origin, is proved by 

 the remarkable fact noticed in the ninth chapter as 

 occcurring in the schist in Plymouth Dock-yard ; 

 where the contortions are marked by differences of 

 colour, and the schistose structure is at angles to them. 



It is not unusual, in the argillaceous schists, for the 

 observer to mistake the direction of the schistose ar- 

 rangement for the plane of the bed. It is true that 

 these are sometimes coincident, as in the secondary 

 schists ; but, in the antient schists, the former is also, 

 perhaps most generally, at angles, often very consi- 

 derable, to the plane of stratification. The mode of 

 making this distinction is stated under the head of 

 argillaceous schist. 



Of the prismatic and columnar Structures. 



The phenomena of decomposition might seem to 

 throw some doubt on the existence of any prismatic- 

 structure different from the columnar, which is com- 

 monly considered as forming a separate division. It 

 is common and well known in granite, and it also 

 occurs in some rocks of the trap family. It is found 

 almost invariably on the large scale, and is possibly, 



