ARCHITECTURE OF FRACTURED SUPERSTRUCTURE 57 



are sometimes added additional though less perfect series of joint 

 planes. 



Studied throughout a considerable district, the various series 

 which make up these two sets of master joints may be seen locally 



FIG. 39. View on the shore at Holstensborg, West Greenland, to show the sub- 

 equal spacing of the joints (after Kornerup). 



developed in different combinations as well as in association with 

 additional fissure planes which are not easily reduced to any simple 

 law of arrangement 

 (Fig. 38 a, a, a). 

 Only rarely are reg- 

 ular joint series ob- 

 served which do not 

 stand perpendicular 

 to the original atti- 

 tude of the rock 

 beds. In a few local- 

 ities, however, rec- 

 tangular joint sets 

 have been discov- 

 ered which divide 

 the rock into prisms 

 parallel to the 



FIG. 40. View of an exposed hillside in Iceland upon 

 which the snow collected in crannies along the joints 

 brings out to advantage both the larger and the smaller 

 intervals of the joint system (after Thoroddsen). 



earth's surface and 



with the joint series inclined to it each by half a right angle. 



Where the rock beds have been much disturbed, the complex of 



