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much of what lias already been said ; as it is found in the same situations, and very frequently in company with siliceous schist. This remark is however chief] v applicable to the secondary division of that rock; and the reason of this will be 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 lmve been differently modified