SILICEOUS SCHIST. 281 



regular gradation of hardness. They never occur ex- 

 cept where such strata are in contact with masses or 

 veins of trap ; displaying- the most perfect characters 

 in their immediate neighbourhood, or where a frag- 

 ment of the strata is entangled in a mass of trap. 

 Where the original shales alternate with limestones 

 and sandstones, corresponding alterations occur in 

 those substances also. The pure limestones become 

 crystalline, while the argillaceous are converted into a 

 species of chert. The calcareous sandstones are also 

 converted into other varieties of chert, as the purer 

 become indurated, or, if much charged with clay, are 

 changed into an imperfect species of jasper ; while the 

 finer clays are sometimes also converted into jasper, as 

 already noticed. If the original shale has contained 

 shells, these are also occasionally found in the siliceous 

 schists ; though often deformed by pressure, or other- 

 wise rendered comparatively ohscure. When the shale 

 has contained bitumen, carbon is also found to he a 

 constituent of the siliceous schist. 



In general, the original form of the beds is pre- 

 "served, even where the change is so complete that the 

 substance cannot be distinguished from basalt ; hut, 

 in other instances, thu loss of form is complete. Hence, 

 irregular and unstratined masses of trap result from 

 the changes which the shales, or perhaps even the pri- 

 mary schists, have undergone ; and this has probably 

 occurred in man peases where we are unable to trace 

 it. The proof is found, as formerly noticed, in masses 

 of trap, or of porphyry, containing imbedded frag- 

 ments of schist, where the progress of the change can 

 also be traced in these fragments : the schistose struc- 

 ture remaining perfect in one part, while, in another, 

 it vanishes gradually into the irregular surrounding 

 mass. And as the primary siliceous schists pass into 

 hornblende schist, so do the secondary ones into basalt? 



