170 



angles more or less acute, and the junction edges are either sharp or 

 rounded. The surface of the sides of the flaw is frequently crumpled 

 or waved, and in the granularly-constituted beds, such as granite, 

 porphyry, or sandstone, is rough, while in slate-clay, bituminous 

 shale, and steatite, it often exhibits a specular polish. 



The circumstance of the flaws exhibiting no approach to paral- 

 lelism, joined to the fact that they are not prolonged into the inferior 

 or superior beds, nay, frequently not extending throughout the bed 

 containing them, furnish a demonstration that they were not pro- 

 duced by an external force. The notion, too, is untenable, that the 

 polishing was produced by the faces of the flaw sliding backwards 

 and forwards on one another, because their limited extent, mode of 

 junction, and waved surfaces clearly indicate the absence of any such 

 alternate shifting. 



The author then stated his opinion that the flaws had been pro- 

 duced by shrinkage, owing to the escape of volatile matter, aided by 

 molecular aggregation, and that the polished surfaces were produced 

 in comparatively soft plastic matter, like bituminous shale, by the 

 presence of water or gas in the cavity, so that the specular charac- 

 ter was the casting or impression of a liquid surface. The empty 

 vesicles of amygdaloid are occasionally found glossy on the walls, 

 or exhibiting an apparently vitrified film, while the rock itself is dull 

 and earthy in fracture. The smoothness in this instance is probably 

 produced as the casting or impress of included vapour or gas. Some- 

 times the flaws in coarse matei'ials, such as porphyry, have a specular 

 aspect, owing to a film of anhydrous peroxide of iron. Illustrative 

 examples were exhibited, and references to various localities around 

 Edinburgh, where the whole phenomena of flawed structure were 

 well displayed. 



2. Observations on the Speculations of the late Dr Brown, 

 and of other recent Metaphysicians, regarding the exer- 

 cise of the Senses. By Dr Alison. 



The object of this paper was to recal attention to the celebrated 

 controversies on this subject, carried on during the last century ; 

 chiefly because some expressions used by Dr Brown, by Lord Jeffrey, 

 Sir James Mackintosh, and M. Morell, convey the impression that 

 the doctrines of Reid and Stewart on this essential part of their 

 system of Metaphysics, are now generally neglected or abandoned. 



