424 .ARRAN. GEOLOGY. PITCHSTONE. 



23. A perfect transition into grey and into brown chert. 1 * 

 I have introduced these transitions here, although they 

 cannot be considered as pitch stones, because they seem 

 to be required for the illustration of the subject. 



24. Dirty yellow green pitchstone, with opake and 

 glistening parts intermixed, and containing numerous 

 spherules, so as to be on the verge of pearlstone, if not 

 actually belonging to this substance. 



25. Dark green; minutely columnar and the columns 

 separating into spheroidal concretions, many of which, 

 perhaps all, contain a central atom of felspar; found 

 loose on Ben huish. The approach of this to pearlstone, 

 and its analogy with the pitchstone of Glamich, are both 

 interesting and illustrative. 



26. A beautiful porphyry containing large and nume- 

 rous crystals of glassy felspar ; the pitchstone basis being 

 bottle green. The peculiar circumstances attending the 

 felspar crystals were already noticed. 



Many of the green varieties become of a pure white 

 on decomposition, desquamating in successive thin crusts. 

 These are opake, and generally earthy in their aspect ; but 

 are sometimes glossy, compact, and brittle, so as to resem- 

 ble common white enamel. The natural green colour of 

 the stone gradually disappears as the decomposition pro- 

 ceeds, as if it was produced by some decomposable or 

 soluble matter. They sometimes become striped in the 

 progress of decomposition, thus indicating an internal 



* The term hornstone having become ambiguous, from its indiscrimi- 

 nate application to different substances, I have thought fit to reject it; 

 the only remedy in all similar cases against future confusion. I am the 

 more inclined to do this, because it was applied before minerals were as 

 well understood as they are now, and because we are in possession of 

 terms more accurate and less liabl^ to misapprehension. All the spe- 

 cimens formerly known by this name, may be referred to compact felspar, 

 or to chalcedony, or J;o chert, or to some transition between the two last 

 substances. 



