188 ARGILLACEOUS SCHIST. 



chlorite are also frequent, but other venous substances 

 are of an accidental nature. Lastly, the argillaceous 

 schists contain nodules or cylinders of the same ma- 

 terial, possessing no internal structure, round which 

 the laminae of the including rock are bent : while 

 these, with other concretionary forms, have led bad 

 reasoners to false conclusions respecting the origin of 

 these schists. 



The transitions of fine clay-slate are into drawing- 

 slate, fine chlorite schist, micaceous schist, siliceous 

 schist, and hornblende schist; the three latter oc- 

 curring only in the vicinity of granite, trap, or gneiss. 

 Those of graywacke are into coarse chlorite and mi- 

 caceous schists, quartz rock, and sundry conglomerates. 

 It passes, further, into red sandstone, where these ap- 

 proximate ; and as it is also a member of that series, 

 though under the name of shale, this has particularly 

 led to the erroneous inference noticed under the Red 

 sandstone, that there was a geological gradation between 

 the primary and the secondary classes. Further, it 

 passes into schistose clays, not to be distinguished 

 from those which occur among the secondary strata. 

 The gradations into gneiss, and into the red primary 

 sandstone, have been already noticed. Organic fossils 

 of various kinds are sometimes contained in argillaceous 

 schist, and even in considerable abundance, as formerly 

 observed: Wales presenting remarkable examples of 

 this occurrence. It also occasionally contains certain 

 crystallized minerals, such as chiastolithe, garnet, 

 staurotide, and others. 



Argillaceous, like micaceous schist, occurs in two 

 modes, either forming huge masses and occupying 

 large tracts, or alternating in smaller proportion with 

 many other rocks, and often in a very intricate manner. 

 In the first case, like the other primary strata, it is dis- 



