Dr. Mac CuUoch on Jasper. 69 



require notice. It sometimes gives indications of a spheroidal 

 concretionary disposition, more or less perfect, and resembling 

 that which, under circumstances of a similar nature, occurs in 

 chert and in siliceous schist. 



In the same way, it sometimes possesses a laminar structure 

 and thus also it approximates to the siliceous schists. It is easy 

 to see how, from similarity of origin, connexions, and compo- 

 sition, it may be thus a matter of doubt to which of those two 

 rocks any given specimen or bed should be referred. The well 

 known striped and spotted jaspers already noticed, owe their 

 appearance to the two structures above-mentioned ; and, occa- 

 sionally, the two are combined in the same specimen. 



It is much more rare to find jasper possessing a minute 

 columnar structure resembling that of the madreporite lime- 

 stone, or of ironstone. But this, when it occurs, is easily ex- 

 plained, when it is recollected that it often differs from this 

 latter substance, only in the degree of hardness. The transition 

 into ironstone is similar to that into the ferruginous clay of the 

 strata which lie under trap. 



The large columnar structure is still more rare, but one re- 

 markable instance occurs at Dunbar, in Scotland ; the columns 

 being of considerable dimensions and resembling, in their general 

 forms and disposition, those so common in the members of the 

 trap family. In this case, the jasper passes into the sandstone 

 and ferriferous shale, or argillaceous ironstone, from which it 

 appears to have been derived ; but as the whole of the circum- 

 stances attending this case are of great importance, I shall 

 communicate a fuller account of them at some future oppor- 

 tunity. 



I know not that it is possible to frame any general description 

 or definition of the characters of jasper by which it could be 

 recognised ; ai least by beginners in this science. It is super- 

 fluous to accumulate characters if they do not conduce to this 

 end, and I shall not therefore attempt it. In a general sense, 

 it may be conceived to be an extremely indurated clay, of which 

 certain varieties approximate in their characters to hard pottery 

 and others to porcelain. It has no predominant texture, and is 



