Crystalline Structures of Rocks. 63 



under similar circumstances ; but that all curvature is not of a 

 concretionary origin is proved by a remarkable fact described by 

 me in the Geological Transactions, occurring in the schist in Ply- 

 mouth-Dock-Yard, Here the rock displays both characters ; the 

 contortions being marked by differences of colour, and the schistose 

 structure being at angles to them. 



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

 mistake the direction of the schistose arrangement for the plane of 

 the bed. It is true, that these are sometimes coincident, as in the 

 secondary schists ; but in the ancient schists, it is most generally 

 at angles, often very considerable, to the plane of stratification. 

 The mode of making this distinction is stated in the work to which 

 I have so often referred. I may finally add, that no mode of ex- 

 plaining the origin or cause of any of the varieties of the laminar 

 structure has yet been suggested. 



Of the Prismatic and Columnar Structures. 



The phenomena of decomposition seem to throw some doubt on 

 the existence of any prismatic structure different from the columnar, 

 which is commonly considered as forming a separate division. It 

 is, nevertheless, necessary to describe it according to the form in 

 which it has generally been understood to exist. It is common 

 and well known in granite, and it also occurs in some rocks of the 

 trap family. It is found invariably on the large scale, and is possi- 

 bly, in these cases, only a modification of the laminar structure pro- 

 duced by fissures. Where it occurs in the sandstones, it appears 

 to be more certainly referable to this cause. When it was thought 

 certain that the spheroidal exfoliation of the cuboids of granite 

 v/as a proof of a spheroidal concretionary structure, it was natural 

 to consider these as prismatic concretions. That will still be true 

 should this be proved ; but as some of the cases of this nature are 

 unquestionably proved to arise from the action of the atmosphere, 

 the whole question must remain open for further enquiry. 



The columnar structure on account of its symmetry and artifi- 

 cial appearance, is unquestionably the most interesting of all these 

 modifications : an interest not a little enhanced by the difficulty of 

 explaining its origin. It presents many varieties, occurs in rocks 



