184 Analysis of Scientific Books. 



paste is very compact, and of a very fine grain. To the 

 naked eye, its aspect is dull; its fracture appears unequal and 

 earthy ; its colour is of a delicate grey. Viewed in a lens, its 

 fracture appears scaly, fine, and its scales are whitish and 

 semitransparent ; it has a waxy lustre ; is hard ; a point of 

 steel, after breaking the small scales, leaves a metallic trace 

 on the stone. It is dense, heavy, and easily frangible. It 

 affords a strong argillaceous odour when breathed upon, and 

 gives feeble sparks with steel. At the blowpipe it whitens, 

 cracks, and finally melts into a white enamel, somewhat ve- 

 sicular. " It is clear," concludes Mr. Necker, " that this rock 

 has been untruly regarded as having a wacke basis, since it 

 possesses none of the characters which distinguish the latter 

 mineral. He considers it as a new product of the trap- 

 formation. It approaches in the nature of its basis to the 

 laves petrosiliceuses described by Dolomieu, in his catalogue 

 of the Lavas of Mtna, p. 254, No. CXI ; and p. 239, No. XXIII ; 

 and by Brochant, vol. ii. p. 627 ; as also to certain compact 

 domites of the rocks of Mont d'Or and of Cantal in Auvergne. 

 This porphyry occurs at Dundou in the form of pretty regular 

 hexahedral prisms, covered with a whitish crust from decom- 

 position. In the lower part of the hill these prisms are ver- 

 tical, whilst on its summit they are horizontal. It is remark- 

 able that those of the columns which occupy the culminating 

 point of the trap-cone, are broken through the middle. The 

 country people aflfirm that this was not the case ninety years 

 ago, and that it was a thunder-stroke which at that period, fell 

 on these rocks, and produced the fractures. 



There is a huge bed of green pitchstone, in a cliff" of red 

 sandstone, at the foot of Dunfeune, which has a beautiful ap- 

 pearance. This mass, three hundred feet long, and twelve feet 

 thick, presents throughout so uniform and smooth a surface, 

 that it reflects the sun-beams like an immense plate of polished 

 metal. On seeing it shine at a distance, travellers might fancy 

 they beheld one of those rocks of solid silver, with which 

 the abodes of the Genii are decorated in Eastern romance. 

 In this district there is a great number of whin dykes (veins of 

 basalt) which intersect the sandstone and breccia in different 

 directions. They are perfectly similar to those which occur on 

 the opposite coast of Ayrshire. The perfect parallelism pre- 

 served by the sides of the vein, or the planes of the basaltic 

 mass, which touches the walls of the sandstone, is very re- 

 markable. Thus many veins or dykes run in a line absolutely 

 straight for several hundred feet, retaining throughout the 

 same thickness. It is also observable, that the sandstone, on 

 approaching one of these dykes, constantly experiences an 

 evident change, both in its colour and consistence. Near the 

 point of contact, it loses its red tint, and becomes harder. 



