ARGILLACEOUS SCHIST. 187 



of the finer and coarser materials, during their deposi- 

 tion, evincing that any alteration between these must 

 he parallel to this plane ; as the facts themselves also 

 prove. If therefore any doubtful bed presents such 

 an alternation, whatever direction its laminar tendency 

 may possess, the plane of stratification must be con- 

 ceived parallel to that ; and by this criterion that 

 question can be determined ; while the proof that the 

 fissility depends on concretionary structure, and not 

 On deposition, in such cases, is evinced by the fact 

 that it not merely crosses the stratification, but is often 

 continued through both the varieties. 



The laminae are sometimes straight and even ; at 

 others, more or less minutely undulated, as in mica- 

 ceous schist; these modifications being most remark- 

 able in the finer clay-slates, though occurring also in 

 the graywackes. Occasionally, they present irregular 

 convexities and concavities ; while this is most common 

 in the graywackes. In a few instances, they possess 

 a fibrous texture, sometimes further combined with 

 the laminar. The coarsest graywackes are sometimes 

 laminar ; at others losing that texture, and becoming 

 massive conglomerates. They occasionally contain 

 grains of felspar, deriving thence a porphyritic appear- 

 ance : and large fragments of quartz sometimes occur 

 in the finer schist, without destroying its fissility. 



All the varieties divide naturally at right or oblique 

 angles to the laminae ; the divisions being commonly 

 filled with ochrey matters, and the joints smooth. The 

 rhomboidal and other forms are familiar ; sometimes 

 producing long beams adapted to architecture. 



Laminae of chlorite and talc occurring, confer on 

 those parts a silky or plumbaginous lustre; as veins 

 of quartz and of calcareous spar are common ; both 

 substances often existing in the same vein. Veins of 



