Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 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 t
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 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
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