7G GEOLOGY OF ARRAN. 



here obscures it, so that its course canuot be traced 

 to the water's edge. Climbing up the cliff to examine it 

 more closely, we find it to be 13 feet 5 inches in thickness ; 

 of lamellar structure and dark bottle-green colour. There is 

 no remarkable change on the sandstone a slight induration 

 merely, but the lower portion of the pitchstone is changed 

 into a blue-coloured, porous, slag-like matter, like a pumice- 

 ous lava : this, however, is probably the mere result of 

 decomposition. Its exact position among the sandstone 

 strata is not easily determined : but here is little doubt that 

 it intersects the beds, and is therefore an intrusive vein. 

 Fallen masses of the pitchstone strew the beach ; and among 

 these, where the path comes close on the water by the base 

 of the cliff, there is a dike of red quartziferous porphyry, 

 nearly perpendicular to the shore, twelve or fourteen feet 

 broad seawards, but narrowing inland to five or six feet 



A granite boulder, about one-third of the size of the one 

 near Dun-an farm-house, rests here in the tideway ; and great 

 numbers strew the shore all along. 



39. Between the two veins of pitchstone which we have 

 just described, there occurs upon the flat shore another large 

 vein of claystone remarkable for a peculiar structure. This is 

 developed in those parts only which are near the junction 

 with the sandstone, along tfte south side of the vein. The 

 appearances are fully and accurately described by Dr. 

 MacCulloch (vol. iL p. 405). The structure referred to " is 

 concretionary globular, or striated the latter being either 

 found separate or united with the globular in the same 

 specimen. The former puts on sometimes the appearance of 

 spots, circular or elliptic, resembling Siberian jasper. The 

 spots, as well as the stripes, are attended with corresponding 

 differences of hardness, the former arising from the globular 

 structure, the latter from a schistose or laminar one. The 

 spots being often elliptical, compressed, or elongated, occa- 

 sionally become laminae in the progress of elongation, passing 

 into them by insensible degrees. The vein next the sand- 



