

ARRAN. GEOLOGY. OVERLYING ROCKS. 405 



be remarked, that, in both these veins, the texture varies, 

 so as to admit in different places of both the terms just 

 used, and that the vein now under review is seldom 

 if ever porphyritic. But the circumstances for which 

 this vein is chiefly remarkable are, a concretionary glo- 

 bular, and a striated structure ; both of these being found 

 in the immediate vicinity of the junction, and no where 

 else. In some specimens, the former puts on the appear- 

 ance of circular or elliptical spots, strongly resembling 

 in aspect the well known spotted Siberian jasper. 1 * The 

 striated variety is either found separate, or united with 

 the globular in the same specimen. In both cases, 

 the spots as well as the stripes are attended with cor- 

 responding differences of hardness ; the former arising 

 from the globular structure, the latter from a schis- 

 tose or laminar one. The spots being often ellip- 

 tical, compressed, or elongated, occasionally become 

 laminae in the progress of elongation, passing into them 

 by insensible degrees. That portion of the vein which 

 lies near the sandstone, and contains these appearances, is 

 very small, but varies much in hardness in different places ; 

 although it cannot be said that this induration bears any 

 relation to its proximity to the sandstone. The concre- 

 tionary structure takes place both in the hard and in the 

 soft varieties ; but with some differences, inasmuch as 

 the globular bodies appear in the latter to be softer 

 than the base, while in the former they are harder. They 



* The term jasper has been used in so vague a manner, and applied 

 to so many substances decidedly different in mineral character, that 

 it is difficult to steer clear of ambiguity in speaking of its geological 

 connexions. But if, in the present state of our knowledge, it may be 

 limited, it will probably be found to appertain chiefly to the overlying or 

 venous rocks of the porphyry family. In this way it occurs in Glenco, 

 |>assing by insensible gradations into the common coloured claystones, 

 and into the porphyry of that district. The well known mass found on 

 the east side of Scotland near Fettercairn, seems also to be an inde- 

 pendent rock intruding among the primary strata, in the same manner 

 us the rocks of thjit family are known to interfere with these. 



