ARGILLACEOUS SCHIST. 345 



variation taking- place, either laterally, according 

 to the extent of the strata, or transversely, accord- 

 ing to their stratification. 



Lastly, this arrangement is justified by the 

 corresponding practice in the case of sandstone 

 which this rock resembles in its prevailing me- 

 chanical texture and composition. In that rock, 

 na geological distinction is made between the 

 varieties of fine and those of coarse texture ; al- 

 though the differences between the conglomerates 

 and the fine sandstones, are far greater than any 

 that occur in the series of argillaceous schist. It 

 will indeed be seen on the slightest review of the 

 former system of division, that it has arisen from 

 defective observations, combined with, or biassed 

 by, hypothetical notions respecting a supposed ne- 

 cessity for placing the coarse argillaceous schists, 

 or graywackes in a transition class. That such 

 mechanically recom posed strata should occur 

 among the primary rocks, is no more cause of sur- 

 prise or difficulty, than that conglomerate varieties 

 of micaceous schist, or of quartz rock, or of red 

 sandstone, should be found in the same situation. 



Argillaceous schist, like micaceous schist. 



