201 



quartz rock, attention be paid to the structure. 

 Into this latter rock it passes not unfrequently, as 

 already remarked ; into gneiss, very rarely ; and 

 thus its characters may become doubtful. 



ARGILLACEOUS SCHIST. 



All the varieties of this rock, however dissi- 

 milar they may appear in a general view, agree in 

 containing stony, or indurated clay, either alone, 

 or as the cement of sand, or of fragments of 

 various kinds. Most of the varieties are more or 

 less perfectly schistose in the structure. If, ac- 

 cording to this description, it is ever in danger of 

 being confounded with the claystones of the over- 

 lying family, and there is no schistose structure 

 present by which to distinguish it, recourse must 

 be had, either to the stratified disposition, or to 

 the geological connections. 



It may, in some cases, be confounded with 

 the harder shales of the secondary class ; and, 

 against this, there is no remedy but an examina- 

 tion of the geological position. 



