AGILLACEOUS SCHIST. 351 



must ever remain a doubt whether this structure 

 is the result of a concretionary arrangement or a 

 mechanical deposition. From the other instances 

 occurring in the same rock, we should be in- 

 clined to determine on the former ; while, from 

 the analogy of the shales, of which the mecha- 

 nical structure is proved by the vegetable re- 

 mains deposited in them, we should decide on 

 the latter. 



A fibrous structure is not unfrequent in the 

 finer varieties of argillaceous schist, and it is 

 often combined with the laminar or schistose dis- 

 position. It may lastly be remarked on this sub- 

 ject of structure, that, in a bed which is princi- 

 pally laminar, there are sometimes found nodules 

 of the same substance, massive and imbedded ; 

 the laminae in the vicinity accommodating them- 

 selves to the form of the nodule. 



Beds of argillaceous schist are frequently 

 divided by natural joints, which are either at 

 right angles, or oblique, to the plane of stratifi- 

 cation. According to these circumstances, they 

 frequently separate into rhomboidal or prismatic 



