186 ARGILLACEOUS SCHIST. 



The general definition of argillaceous schist is, an 

 indurated schistose clay, either simple, or mixed with 

 quartz, occasionally, with mica also, and with other 

 ingredients. In Clay-slate, no quartz is visible ; in 

 Graywacke, the basis of clay-slate may contain mica 

 only, or quartz sand, or both ; and also gravel of dif- 

 ferent kinds, becoming at length a conglomerate of 

 various fragments or pebbles, united by indurated clay. 

 The details of these, and of the varieties termed hone, 

 whetslate, &c., will be found in the Classification of 

 Rocks. 



The internal structure of argillaceous schist is va- 

 rious, though not always depending on the fineness of 

 the ingredients. The schistose, or laminar one, is 

 more or less perfect, in both varieties. In clay-slate, 

 it has no apparent cause ; but, in graywacke, it often 

 depends on the occurrence of a micaceous lamina, or 

 of minute scales of that substance. In such cases, the 

 laminar character depends on the gradual deposition 

 of the materials ; while the analogies of micaceous 

 schist, gneiss, and the fissile sandstones, would lead us 

 to infer that it was always parallel to the plane of the 

 bed. That, indeed, is often the case, particularly 

 where thin beds of this rock alternate with micaceous 

 schist, quartz rock, or gneiss. In other instances, 

 however, it is not conformable to the stratification, 

 being placed at different angles towards it, and, some- 

 times, even transversely. When the beds themselves 

 are exposed, this fact is easily determined ; while, if 

 the direction of the fissility is alone seen, an erroneous 

 judgment is often formed respecting the real positions 

 of the strata. If there are sometimes no means of 

 correcting this judgment, the real stratification can, 

 at others, be ascertained by examining the internal 

 structure ; as I long since showed to those who have 

 committed this frequent error : the mode of separation 



