74:8 JOINTED STRUCTURE AND CLEAVAGE. [Ch. XXXVI. 



quarryman, as Sir R. Murchison observes, when speaking of the phe- 

 nomena, as exhibited in Shropshire and the neighboring counties, 

 the greatest aid in the extraction of blocks of stone ; and, if a sufficient 

 number cross each other, the whole mass of rock is split into symmet- 

 rical blocks. The faces of the joints are for the most part smoother 

 and more regular than the surfaces of true strata. The joints are 

 straight-cut chinks, often slightly open, often passing, not only through 

 layers of successive deposition, but also through balls of limestone or 

 other matter which have been formed by concretionary action, since 

 the original accumulation of the strata. Such joints, therefore, must 

 often have resulted from one of the last changes superinduced upon 

 sedimentary deposits.* 



In the annexed diagram (fig. 759), the flat surfaces of rock a, b, c, 



Fig. 759. 



Stratification, joints, and cleavage. 

 (From Murchison's Silurian System, p. 245.) 



represent exposed faces of joints, to which the walls of other joints, 

 j j, are parallel, s s are the lines of stratification ; d d are lines of 

 slaty cleavage, which intersect the rock at a considerable angle to the 

 planes of stratification. 



In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enor- 

 mous masses of limestone are cut through so regularly by nearly ver- 

 tical partings, and these joints are often so much more conspicuous 

 than the seams of stratification, that an inexperienced observer will 

 almost inevitably confound them, and suppose the strata to be perpen- 

 dicular in places where in fact they are almost horizontal.! 



Now such joints are supposed to be analogous to the partings 

 which separate volcanic and plutonic rocks into cuboidal and pris- 

 matic masses. On a small scale we see clay and starch when dry 

 split into similar shapes ; this is often caused by simple contraction, 

 whether the shrinking be due to the evaporation of water, or to a 

 change of temperature. It is well known that many sandstones and 

 other rocks expand by the application of moderate degrees of heat, 

 and then contract again on cooling ; and there can be no doubt that 



* Silurian System, p. 246. f Introduction to Geology, chap, iv 



