CHERT. 285 



much of what has already hern said ; as it is found 

 in the same situations, and very frequently in company 

 with siliceous schist. This remark is however chiefly 

 applicable to the secondary division of that rock; and 

 the reason of this will he found in the generally supe- 

 rior purity of the primary limestones; whence the only 

 change they can experience from granite or porphyry, 

 is that of crystallization. As however the primary 

 limestones are occasionally intermingled with siliceous 

 and argillaceous matter, either in a state of minute 

 mixture, or in that of a laminar alternation, chert is 

 sometimes found even among these. An attempt to 

 the production of this rock is also often observed where 

 the process is still incomplete; and it is evinced by the 

 extreme hardness which such limestones exhibit in the 

 vicinity of granite; but no decided instances of an ex- 

 tensive and perfect transition from these into chert, 

 has yet occurred within my experience. From the 

 peculiar manner in which granite comes into contact 

 with the stratified rocks, distinct beds of this substance 

 cannot be expected in its vicinity. The change to chert 

 in the approximate limestones, is therefore irregular, 

 and limited to short distances from the planes of con- 

 tact. This is visible in various parts of Glen Tilt, in 

 Shetland, and in other places. It is easy to conceive 

 that beds of this rock might be formed by the contact 

 of overlying masses of porphyry ; but I cannot quote 

 any facts to the purpose. 



In the secondary class, the origin of chert is more 

 easily traced, and the quantity is, at the same time, 

 much greater. This arises, partly from the more ar- 

 gillaceous nature of the limestones in this division, 

 and partly from the mode in which the trap rocks in- 

 trude among the strata, in parallel veins. In what 

 respect the heat may huve been differently modified 



