SHALE. 457 



in some, very conspicuously. In these cases also, 

 it is often very difficult to distinguish it from the 

 finer graywackesof the primary series ; while, in 

 some instances, it is even easily confounded with 

 clay slate, as formerly remarked, unless examined 

 in conjunction with its geological connexions. 



It occurs in many of the secondary limestones ; 

 either alternating with the calcareous strata in 

 considerable beds, or forming thin laminse,or else 

 entering as a constituent with other substances, 

 into some of the peculiar groups in which many 

 of these are found 



Shale is also found associated with coal. In 

 this case, it either alternates with the coal itself, or 

 forms a component part of the peculiar series of 

 which that substance is the most conspicuous 

 member. 



Further, it is found among the clays ; some 

 portions of these important deposits appearing to 

 have been indurated into this form, while others 

 have retained their loose earthy character. The 

 enumeration of its associations already given in 

 the two preceding articles, renders it unnecessary 

 to take further notice of them here. 



