*c 



ie 



an 



I 



a 



»rf It 



Re 



«i 



18 



tidik; 







Sil 



pn 



# 



: /< 



i 



I 



i 



■ 



/< 





JOINTS. 



121 



rently been interrupted by an alteration in the fineness of ITrraa 

 the grain of the rock at the end marked x . Galubby. 



13.— Purple slate and grit, as above. The bedding WaU -<»»e48. 



of slate 



front and 

 the regularity and 



smoothness of the cleavage being interrupted where it enters 



the grit. 



coincident with the bedding. 

 joints. 



fracture 



In all 



14.— Banded slate and fine gritty bands, as above. 



In this specimen cleavage and bedding probably coincide. 

 The numbered side and its opposite are in these planes. 

 The other four planes are joints producing a rhomb. 



15 to 24.— Purple Cambrian coarse slates, as above. 

 " these the original muddy purple substance of the 

 rock is so homogeneous that the bedding in small spe- 

 cimens is usually indistinguishable. The beds from which 

 they come are exceedingly contorted, and the cleavage 

 nearly vertical, so that cleavage and bedding only acci- 

 dentally coincide. On the specimens the figures are marked 

 on the cleavage planes. All the other planes are joints, 

 giving various forms to the specimens. 



In No. 15, the end joints if prolonged would form a tri- 

 angle with the others. No. 16 is a rhomb, and nearly in 

 the same planes as those on the right and left there are four 

 joints in the body of the specimen marked by faint white 

 lines filled with silica. 17 to 20 are also rhombs. The 

 joints of 21 and 22 form scalene triangles, the latter im- 

 perfect from the interference of a small joint at the base. 

 The joints of 23 orignally formed a triangle, but one of 

 the angles has been knocked off at another joint so as to 

 form a four-sided figure. In 24, the planes of the joints if 

 produced would meet at different points, and the same is the 

 case in other specimens. 



25 — Conglomerate, Cambrian rocks, St. Davids, Pem- 

 brokeshire. Map 40. "" 



pebbles. 



Planes of joints cutting through the 



