Geological Society, 227 



minerals, except opaque white quartz forming numeroul., 

 veins. Nearlv the whole of north Devonshire is composed 

 of the rock just described, which is locally distinguished 

 into dimstone and s/iillat, the latter being the slaty grau- 

 WiTcke, and the iormer the compact. It is always very irre- 

 gular in Its stratification, is destitute of metallic veins, alter- 

 nates with transiiioti limestone, and, where it does so, occa- 

 sionally contains organic remains : it also, in one instance 

 at least, alternates with thick beds of a kind of culm; its 

 veins, l)esides quartz, occasionaUv offer calcareous spar. 

 Killas, which Mr. C. is inclined to regard as a variety, not 

 of mica slate, but of clay slate, is contorted in its stratifica- 

 tion only in the neighbourhood of the grauwacke, is tra- 

 versed almost through its whole extent by frequent veins or 

 dykes of a porphyritic rock which does not pass into the 

 grauwacke, contains soinelimes topaz, and not unfrequently 

 garnet: it>! veins are often filled with chlorite mica and 

 crystallized feUpar, and also contain tin stone, gray cobalt 

 ore, &;c. These characters, in the opinion of the author of 

 this paper, form a sufficient mark of distinction between the 

 grauwacke and the killas of the West of England. 



A paper on the island of Staffa, by Dr. MacCulIoch, was 

 read, and thanks were voted for the same. 



The circumference of this island is about 2 miles ; it 

 forms a kind of table land of irregular surface, gently 

 sloping to the N. E. and is hounded on all sides by steep 

 and generally perpendicular cliffs, from 60 to 70 feet above 

 hijrh-water mark, the greatest elevation in the island being 

 about 120 feet. 



The entire island is a mass of basalt, but a bed of sand- 

 stone is said to be visible al low water on the western 

 side. 



The basalt presents two varieties, the columnar and 

 amorphf)us, the latter of which is generally amygdaloidal, 

 containing zeolites. 



In the columnar variety lamellar slilbite is occasionally 

 found filling the intervals of approximate columns, and 

 sometimes, tliough rarely, in the substance of the smaller 

 and more irregular columns. 



On the south-western side of the island there appear to 

 be tlirte distinct beds of basalt, the lowermost of which 

 seems to be amori)h<)us. Tiic next bed, from thirty to 

 fifty feet ihiek, consists of those large columns v\hieh form 

 the conspicuous feature of Siaffa : the upper one appears, 

 at a distance, lo be a mass of amorphous basalt, but on 

 closer inspection is found to consist of small columns, often 



P 2 wood- 



