118 ON THE FLEXURES AND 



observer may trace these deceptive prolongations 

 to the stratum to which they belong. 



There are instances in which the contortions of 

 rocks are not at first obvious to inspection, although 

 they admit of being discovered. This sometimes 

 happens in the case of the schists, which show signs 

 of it only after they have been exposed to the action 

 of heat; if these are notj-ather examples of the con- 

 cretionary structure. In other cases, where contortion 

 and a concretionary structure are co-existent, the 

 former may be concealed by the latter ; or, while the 

 indications of concretionary structure are marked in 

 the divisibility of the rock, those of the contortion are 

 pointed out only by variety of colour. A mass of soft 

 schist in the Dock-yard at Plymouth, offers an acces- 

 sible example of this fact; the contortions being 

 indicated by an alternation of different colours at 

 angles to the schistose structure. 



Although the most important and remarkable cases 

 of contortion are found in the strata, the unstratified 

 rocks are not exempt from them, and they require to 

 be noticed among the others. From the very nature 

 of these rocks, it is evident that they cannot be easily 

 seen ; nor indeed are they to be discovered except 

 from the consequences of decomposition. They are 

 limited to pitch stone and to some members of the trap 

 family. In the former, they are discovered by dif- 

 ferences of colour on the surface, resulting from the 

 action of the air ; and, in the latter, they are commonly 

 rendered evident only by a further advance towards 

 decomposition. They will require no other observa- 

 tions here ; as they are noticed in treating of these 

 rocks, and as their causes are entirely distinct from 

 those which have produced the corresponding- effects 

 that are found in the strata. 



