
Parr IL] JOINTS IN MASSIVE ROCKS. 505 
quarried out entire. Such monoliths have been from early times 
employed in the construction of obelisks and pillars. 
_ In large masses of granite an outward inclination of the natural 
divisional planes of the rock may be sometimes observed, as if the 
granite were really a rudely bedded mass having a dip towards and 
under the strata which rest upon its flanks. It is not a foliated 
arrangement of the constituent minerals analogous to the foliation of 
VS" 
NWN 
ANAS 
\ WS SS \ 
Zs 
* 

Fic. 220.—PorpHyry, NEAR CLyNoG VAwR, CAERNARVONSHIRE, DIVIDED INTO SLABS 
BY A SYSTEM oF CLOSE PARALLEL JOINTS (B.). 
gneiss, for it can be traced in perfectly amorphous and thoroughly 
crystalline granite, but is undoubtedly a form of jointing by reason of 
which the rock weathers into large blocks piled one upon another 
like a kind of rude cyclopean masonry.* 
Rocks of finer grain than granite, such as many diorites and 
dolerites, acquire a prismatic structure from the number and inter- 
section of perpendicular joints. The prisms, however, are unequal in 



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Fig. 221.—JomsteD STRUCTURE OF GRANITE. 
dimensions, as well as in the number and proportions of their sides, a 
frequent diameter being 2 or 3 feet, though they may sometimes be 
observed three times thicker, and extending up the face of a cliff for 
300 or 400 feet. It is by means of joints that precipitous faces of 
crystalline no less than of sedimentary rock are produced and 
retained, for they serve as openings into which frost drives every year 
* In the granite of the axes of the Rocky Mountains and parallel ranges to the 
westward, a kind of bedded structure has been described as passing under the 
erystalline schists. 
