Scientific Intelligence — Geology. 363 



GEOLOGY. 



8. On the Structural Characters of Rocks ; by Dr Fleming. — 

 While the condition of the mineral masses in the neighbourhood 

 of Edinburgh furnish interesting illustrations of the structural cha- 

 racters of rocks, such as the columnar, the concretionary, and the 

 fragmentary, &c, the author proposed to confine his remarks at 

 present to what he denominated the Flawed Structure. 



In the ordinary language of quarriers, the flaws are termed backs, 

 while they are known to masons as dries, and to geologists, when 

 referred to, as slicken-sides. This last term, independent of its 

 provincial character, refers to one peculiar form of the flaw only, 

 and, although explicable according to the same views entertained 

 respecting the origin of the others, is far from being a typical form. 

 The flaw of the lapidary, in reference to crystals or gems, comes 

 sufficiently near in character to justify its adoption. 



The Flaw is a crack which is confined to the stratum or bed in 

 which it occurs, and is thus distinguished from fault or dislocation, 

 since these extend through several beds. It occupies all positions 

 in the bed, without an approach to parallelism, the flaws being 

 variously inclined to one another, and not extending continuously 

 throughout the thickness of the bed ; thus differing from the colum- 

 nar structure. 



These flaws are sometimes isolated ; in other cases two unite at 

 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 in- 

 ferior or superior beds, nay, frequently not extending throughout 

 the bed containing them, furnish a demonstration that they were 

 not produced 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 ex- 

 tent, mode of junction, and waved surfaces clearly indicate the ab- 

 sence 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 pro- 

 duced in comparatively soft plastic matter, like bituminous shale, 

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

 character 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 vitrin:d film, while the rock it- 

 self is dull and earthy in fracture. The smoothness in this in- 



