mo 
¢ 
Parti] JOINTS IN STRATIFIED ROCKS. 503° 

regular surfaces, and is known as the end. The face remains persis- 
tent over wide areas; it serves to define the direction of the roadways 
in coal-mines, which must run with it. 

























































Fie. 218.—JomnTING IN QUARRY OF CAITHNESS FLAGS, NEAR HOLBURN HEAD. 
The cause of jointing has not been satisfactorily explained. 
According to observations made by Jukes, both strike-joints and 
dip-joints occur in beds of recently formed coral rock in the Aus- 
tralian and other reefs. These masses of calcareous sediment have 
certainly never been subject to the pressure of any superincumbent 

ONE FOOT 

Fig. 219.—PuAn oF CoARsE CONGLOMERATE OF Buocks OF CAMBRIAN Rocks IN 
CARBONIFEROUS LIMESTONE, TRAVERSED BY A LINE JoINT CUTTING THE INDIVIDUAL 
BovuLDERS IN THE LINE a b, Coast NEAR SKERRIES, DuBLIN County (B.). 
rock. Their joints may possibly be due, as Jukes believed, to con- 
traction during consolidation.1 But in many cases the existence of 
joints points to some much more potent cause than mere internal 
+ “ Manual of Geology,” 3rd Edition, p. 184. 
