TUANSACTIONS OF THE SECTIONS. 655 



the ooliticj coal, and mountain limestone strata, in the latter of 

 which he has endeavoured to ascertain what relation of direc- 

 tion, intersection, width, and other characters there may be be- 

 tween joints and mineral veins. 



Geologists have in general given little attention to the joints 

 in stratified and especially secondary rocks, though the structure 

 which they impart to these rocks is often quite as remarkable 

 and characteristic as that derived from the stratification. In 

 most instances they cause in rocks of a given mineralogical cha- 

 racter a definite figure of the separable blocks ; as the prisms 

 of basalt, the tables of slate, the rhomboidal masses of shale, 

 the cuboidal blocks of limestone : they are often arranged with 

 so much regularity as to assume some of the leading features of 

 symmetrical crystallization. There are several kinds of joints. 



1. Cracks, which are usually confined within the substance 

 of one bed of stone, and appear to have been caused internally 

 by the process of condensation of the mass. Some of these, 

 called by the workmen tlrt/ cracks, though scarcely visible to 

 the eye, open with slight blows, and often display on their faces 

 dendritical oxide of iron, or oxide of manganese. The width of 

 the space left between the opposite faces of a crack varies in 

 some ratio to the nature of the rock ; thus in some septariate 

 ironstone beds and nodules they are very wide, in certain lime- 

 stones almost evanescent. Some of them are empty, others 

 filled with carbonate of lime, quartz, and other substances, 

 and in particular places with sulphuret of lead, carbonate of 

 copper, &c. 



That the cracks have been produced since the deposition of 

 the rock is easily proved, for they divide imbedded shells, plants, 

 fishes, &c. ; in conglomerate rocks also they are found to divide 

 the rolled masses, as at Oban and in the Righi, where veins of 

 quartz filling cracks traverse many different sorts of pebbles. 



2. Joints which go through one bed of stone, or even through 

 several of the conformable beds of the same quality, in several 

 directions, dividing them into blocks of characteristic forms. 



3. Fissures traverse a great variety of strata, though of very 

 different quality, as limestone, shale, sandstone, and coal ', these 

 are often termed backs, or slines ; they, as well as the other 

 joints, are of great importance to quarrymen and miners ; they 

 materially influence the lines of the escarpments of rocks along 

 hill-sides and valleys, the direction of streams, &c. 



In some rocks, e. g. magnesian limestone, they are open for 

 great lengths and depths, or filled with clay and pebbles, intro- 

 duced from the surface : in the oolite and mountain limestone 

 they exhibit every gradation of spariy repletion from a few cry- 



